Glad to see that your site is back online.
With the above, I would advise the first thing you do is to check each to ensure they are up to date, before you start enabling them. Having so many, its very likely you will hit an issue at some point. If you do and cant get back to the ACP you can use recovery mode to get back to the ACP at any point
https://invisioncommunity.com/4guides/welcome/getting-support-r292/
Its worth also reviewing what you did to get to this point, just to save you having the same problems again in the future, as I do understand it can be very frustrating
Ensure you are up to date as releases come out. This will prevent these kinds of issues from occurring as much. In this instance, an app scanner was added in earlier versions where switch to PHP 8 was nessesary, and this version was compatible with both PHP 7 and PHP 8. So it would have told you there was an issue prior to upgrading. Unfortunately, because you had never upgraded, you never got the app scanner before it was too late.
Ensure your 3rd party items are up to date at all times. This is just as important if you are going to use them, as it is keeping the main applications up to date. Many are likely to have released versions compatible with PHP 8 since the versions you have
Use 3rd party items sparingly - If you have 50 3rd party applications, I would really go through those and see if you need them. 50 is a lot for any site
The support area of your admin CP, will more than often be correct. It stated there that your files were out of date. You resolved this by uploading the files again and running the upgrader.
If you have uploaded files, you would always need to have run the upgrader.
Not a dig in any way. Just trying to help you to understand where you have gone wrong, so you can prevent yourself the frustration in the future 🙂