There are likely only really 2 possibilities. The first is that something got corrupted during the download of the update resulting in actual, faulty files. This is highly unlikely as any such event would usually just stop everything dead. But there is always just that remote possibility.
The more likely and more common causes are simple script conflicts. And, of course, I can not look for those as you do not have SP running. You can, however, by simply using your web console. So you could activate the forum, run a few pages and try a few tasks and note what is reported in the console. This FAQ will guide you on how this works. It is very easy – https://simple-press.com/documentation/codex/faq/how-to/how-to-use-your-web-browser-console/
You clearly have tried the ‘remove all plugins and add then back one at a time’ process – which is always a good test. If you can do that again – but this time activating Simple:Press first – ir might also prove useful.
Also – has anything else changed? Most people usually say no to this but it more often than not proves to be that quite a few things have been added, updated etc., and any such change can have knock-on effects.
And finally… we have had issues with Avada before. The last seemed to be connected to them loading an unsupported JavaScript library that clashed with the standard, supported and supplied WordPress libraries but I would not expect that to clash with the admin side of things.
So – I do not mind taking a closer look at your site but would (a) need to have the forum active and (b) if you wanted me to look on the admin side of things, would need an admin account so that I could log in.