well "just" bridge in carla with that and we have access to pretty much all plugins we want.
Carla has a native API that allows to be reused inside other tools, Zrythm is already making use of this.

So many possibilities, so little time...

Is the audio engine up to par with more recent softwares or is it irrelevant?

no idea, that would require digging a lot deeper.
It is an old project that has been barely been kept up to date, so I would guess not much... but it is pure speculation of course.

I'd love to support a second FOSS audio editor. If you ever want to tackle this, I'd support however i can

that is very appreciated, but a few people alone are not enough.
I would love to work on these things full time, but without a massive audience for support, it won't happen.

So for now I can help on maintenance, providing a way to do automated builds.
Someone else on IRC is trying to get a few more things working on the Qt5 build.

Are you familiar with AppImage or similar frameworks? Or do you know anyone who is? (and is willing to help, of course)

Reviving an abandoned Qt program seems like a better idea than forking Audacity.