And there are plenty of libraries and frameworks that can be pulled in to handle things like UI, compression, etc. The OP uses imGUI which is an excellent, small UI library for making in-game editors.
When choosing to go down that path you're not making, "an engine for all games." So there is a ton of work you're not doing.
And every external libary you do pull in, to ease some of the workload, is just waiting to go abandonend next year. So instead of focusing on release by that time, you will now focus on reinplementing that needed functionality that just stopped working.
There’s a spectrum of options here.
That makes no sense. A library being abandoned doesn't mean it suddenly stops working.
Using abandoned tools either means you're very sure all use cases are covered, or that your own engineers are willing to hack around should it not be sufficient. And I think anyone who works with legacy code knows that navigating already written codebases without guidance can take just as long as whipping up a custom implementation.
The web evolves. Lots of features get deprecated all the time and sometimes removed or change behavior in a significant way.
Unreal Engine is really the Unreal Tournament/Gears of War engine at its core.
Good engines are those that are focused on what the game needs. YAGNI/KISS apply here too