But you are most certainly correct that swapping out the physics engine is not something to be done on a whim, it’s a major undertaking.
If you want to use PhysX, you can just use Unity or go back to Unreal 4. Those are the easy solutions.
And using an older version of Unreal is not necessarily bad there are plenty of people sticking with 4. A lot of the changes coming with 5 are a bit too much and a bit too fast sometimes (e.g. pushing out half-broken stuff, like chaos flesh which is completely unusable, chaos physics which are problematic as is being discussed here, some old good features removed to make way for new ways to implement them in some future version, yeah, too much too fast; it’s been severely lacking in quality control).
Nanite and Lumen are not essential features, plenty of amazing games were made without them, Lumen has some pretty serious issues (basically can’t get rid of ghosting when using it) and nanite is cool but you can manage without it.