First of all: I was one of the moderators of phpbb.de and also involved in the development of phpBB many years ago which is why I presume this judgment here
I think the current phpBB has not only a problem with the frontend.
I myself have thought about it several times in the last few months and also wanted to start getting back into forum development. Unfortunately, I can't find the current "philosophy" of the board as it used to be. It seems to me that by hook or by crook they are trying not to cause any breaking changes so that plugins or other technology will continue to work.
Also technologies (e.g. the wiki which was outdated and now finally deactivated) are kept active before thinking about replacing them with more modern ones.
Maybe you should think about "cleaning up" the staff and recruit new developers. One reason why I rather dislike the phpBB code currently is that the code base contains a lot of self-built code instead of relying on components from established frameworks like Symfony, Laravel or Yii. I would also throw overboard legacy support (PHP <8.0; anything Bootstrap (frontend) doesn't support), etc . And furthermore, as mentioned above, risk that with a new codebase, the plugins all have to be rewritten as well. Along with technologies like phpstan, phpmd, phpcs and a clean CI/CD it would certainly move forward with phpBB again.
Don't get me wrong Marc, I think it's great that you are still actively programming on phpBB as far as your time allows but if I look at the commit history in Github you are one of the few active developers but currently phpBB is just vegitating and other forum software like Woltlab (commercial) or SMF are taking users from phpBB to themselves.
And now hit me for my statements