There are so many questions that need to be answered before even deciding how to proceed. That said, the data structure is the most important, and getting that right is probably the important foundation of the entire project. That said, in my opinion the data lacks some items, and some items can be streamlined. I'm sure we are going for multiple rounds on this, so here we go with round one:

Please also note that I have only thought about this for only 10-15 minutes and not days/months...

Data Structure:
1. Is each Artist supposed to have full data access - meaning will each one be able to manipulate the data (add/edit/delete)? If this is the case, there are some serious issues here, you will need to have elaborate User Permissions scheme, and by just looking at the database you created, there is none - yet.
2. Depending on the technology to be used to create the project (more on this later), you can streamline the table a little bit, for example you can get rid of the Artists table, and include the Artists in the Band table. HOWEVER, because different bands have different Artists, you will need a "Repeater Field", where you can add any number of Artists. An example of a Repeater field is an invoicing app, where you can keep adding as many items to it as you want.
3. With the Repeater field you can also get rid of the Playlist Song table and merge that with the Playlist table.
4. Likewise, you can have a Radio button for the Playlist table to decide whether the playlist is for rehearsal, gig or both, and depending on the choice(s), you will then have fields to add their corresponding details (name, date, time, etc...)...

Technology:
You really have 4 main choices here:
1. Are you trying to build this from scratch, ie: a complete standalone app, or even build it on top of CMS like QuickerSite? Besides the main data functions, you will need User Permissions functions, as well as templating and almost most important, a filtering system to display the data depending the users input, Language, Genre, Time, Band, Playlist, etc...
2. If you are willing to spend a few hundred dollars a year, you can speed up production time with ASP/PHP/.NET Runner, or ASP/PHP/.NET Maker. However, the front display you will get out of the box will look more like a database app., as oppose to a professional website.
3. Third choice (and you will hate this!!!), you can use WordPress with Advanced Custom Pields Pro plugin, and a Full Site Editing Theme. A FSE theme is basically a theme with a JSTemplate/Artisteer like features built in. Once done and packaged the theme after development, you can take the 2 zip files, the plugin and theme to any website, and right away start adding the data right away. Also, you will have elaborate filtering system, User Permissions system, no coding templating system, the choice of front end and/or backend editing, and a fast professional looking site, that will perform in about 0.5/1 second after all said and done..., BUT with a free third party Cache plugin the pages will load immediately, and I mean immediately. You can also decide what piece of info can be seen on the front end, and what is just for backend stuff, like Rehearsal dates, and Playlists, etc...
4. Finally, have AI code for you. I tried many of them, and when it comes to coding, unless it's something very simple they all suck, ChatGPT, GitHub Copilot (which is ChatGPT btw...), Bart, Perplexity AI, etc... maybe in a few years or so...

If you build this with php, you can possibly find some band managers to sell this to, or to bands as a software as a service type of a site which you will have to create and maintain, and getting rid of the multiple bands feature and make it for one band, then you target market will be individual bands, which is a much bigger marketplace.

Nick, awesome!

1. Sure we need security and permissions. The main key-issue is how to give permissions to band-members. Some band-members may not have access to some playlists. Others can see all playlists. I would stick to one main user for editing/adding/removing x-number of bands. The solution should be able to hold an unlimited number of users though (a bit like QuickerServer held an unlimited number of QuickerSites).

2. I plan to build this on top of ASPlite, the classic ASP developer framework I have been working on for some years. So yeah baby, it's gonna be classic ASP and nothing else. For front-end I'd use Bootstrap and a bunch of jQuery plugins (DataTables, CKEditor, JSZip, jQuery UI, etc.)

3. I am not thinking about selling it at this point. To be honest, I need this for myself in the first place. I make money doing gigs these days. So if this would help me to organiser my "job", I'm sure it's gonna help some other musicians too. At this point I'm not sure about redistributing code though... I guess I'd better offer this as a hosted service... What do you think?

4. I have used ACF in WordPress before. It's an amazing app. But the underlying datastructure is terrible, which is a big issue as soon as you're dealing with 100.000's of records. Things get too slow. Pages take >3 seconds to load as as you start using this app the tough way.

5. I stay away from AI at this point. I'm the craftworker-type of developer. I believe those final 10-15% of finishing touches in an application often make a big difference. You never get these with codebuilders, AI or application-generators.

Sorry for the delay, you are absolutely correct about slow queries when you have 100k records, and with ACF you can reach that many records in a relatively short time, if you have to many fields and too many tables (in WP language: too many Custom Fields and too many Custom Post Types). And when you have queries a cache will not help you. The reason is that the data of each custom field in WP is not stored in the same record as the post, but in a separate table and each custom occupies 2 records per post.

For example. in a regular database if you have 5000 records in a table for posts, and it does not matter how many fields, you will end up with 5000 records.

Now, let's take an example of a Real Estate project where you will have 25 fields to hold various data regarding each property, (address, price, size, amenities, etc...). and lets assume the same amount of records from above, 5000 posts: In WP you will end up with:

- 5000 records for the post, and,
- 25 x 2 x 5000 = 50,000 records for the custom fields,

Giving you a grand total of 55,000 records as oppose to the just 5,000.

A non shared hosting will help you a bit, but still the delay in your filter queries will be noticeable.

HOWEVER, there are 2 plugins that I know of, one is a commercial plugin from Australia, and a free one at wordpress.org, that will change how custom fields are saved, and will use a separate table and save all the data in one row, so you will end up in this case with just 5,000 records.

The info to the problem and solution is here - https://www.awesomeacf.com/storing-acf-data-custom-database-tables/ and the actual plugin page is found here: https://hookturn.io/downloads/acf-custom-database-tables/

Just remember this, there must be a reason why WP now runs over 40% of the entire Internet. For large busy sites for me the issue is money, since each mysql database can only handle about 150 concurrent connections, the actual reported number though is more close to 120. Getting the commercial version of the database which is owned by Oracle, was $32,000/year when I last checked in 2012, which apparently can handle 32,000 concurrent connections. The other choices are using Oracle, which now we are talking about quarter of a million per year, or cloud hosting, where you will be using multiple databases and need database load balancing schemes, which I'm guessing that the hosting companies will do that now.

I'll have more over the weekend, I will be out the next 2 days...

I just realized my calculations above were WAY off (long day). Let me try again:

-5000 records for the posts, and for the custom fields,
- 25 x 2 x 5,000 = 250,000

Grand Total = 255,000 records for 5k posts with each post having only 25 custom fields.

P.S. Don't call me Joe Biden yet, I caught my mistake on time !!!

Forgot to mention, the free plugin is at https://wordpress.org/plugins/acf-to-custom-database-tables/

Sorry for the delay Nick. If your calculations are correct, I don't think ACF is a serious option... such an outrageous database overhead. Also, I don't feel very safe with a custom plugin, fixing another custom plugins mess, inside a WP that itself is also changing direction every now and then... No offense, but this does not work for me.

I think I'm gonna go for the datamodel attached. I added band members and user-languages for the interface. I will keep you posted. Many thanks again buddy! So happy to hear from you again!