

So this is kind of going to be weird advice. Maybe not exactly what you are looking for.
You don’t need all those synths and plugins. You really don’t. You can get so, so far just by learning to use the tools that are built into Reaper or Bitwig.
There are really only a handful of audio effects that “really exist” a lot of FX plugins are just complex chains with fancy UIs. But if you understand what they’re doing “under the hood” you can reproduce almost any sound from from the basic building blocks you have.
The same is true for synthesisers. You truly only need one synth as long as it is a flexible, general purpose modern synth. Learning to use one specific synth to it’s full potential will let you make almost any sound you can imagine. Vital synth is a perfect candidate here btw.
Overall I kind of think part of the Linux mindset in general is kind of about doing things your self based on an understanding of the core principals at play.
That being said, there are tons of plugins coming out with native Linux support all the time and it’s getting better every day.
I’m actually about to drop some Linux builds of some plugins myself but I don’t want to dox myself here so that will have to remain a mystery for now.
This is a thing. It just doesn’t work very reliably for the reasons you mentioned.
It does work kind of ok on some comics like the one OP shared and generally pretty well on manga since that tends to be more consistent with the boxes.
I don’t know of anything open source or self hosted that does this though.