This is likely your fault - but not really. Hear me out.
Anyone working on a project for release should know better than to trust epic to begin with - this is where your fault really is.
Then, there is the fact that epic releases things that aren’t market ready while making everyone think they are ready to go. They do it so often and so well that its only normal that people who aren’t experienced fall for it.
What’s worse is that they don’t really warn you when you update a project between engine versions either - Epic Assumes you know.
And really, everyone should know - but it isn’t a given…
As to why you are the only one trying to fix this mess, I doubt you are alone on it (because its been a long while).
However, most people who are near ready to publish are likely properly set up in a source version of the engine which they previouly tested for viability.
And when the chose it, it was a viable release and not a “preview”, so in the 7 or so months after the updates it got (in the repo, not on the launcher) would have only made the baseline better…
If i were you, I’d move to whatver git version can be found where things work after testing.
It’s a lot of work, no doubt - but its likely less work than fixing the engine? Especially if you have no idea how or what you need to do…
My guess on that is that the render pipeline is compromised by newer stuff that requires DX12.
You could try to push the game in forward rendering and see if at least that ends up compiling/looking half decent…