It took some time for me to find a way to use my mars2 printer without using any proprietary software, so I figured I would post about it for other people to whom it matters. The goal here is to replace Chitubox/Lychee used on my computer, I did not mess with the printer itself (meaning: this is not about flashing the printer's firmware with a libre alternative).
The slicer I use is prusa-slicer. As far as I know, it's the only libre slicer handling SLA. Funny enough from what I read, some people already use it for automated support, but then they export the result as STL and import it in Chitubox, the reason being that prusa-slicer generates SL1 files, while the mars2 needs CTB files.
But then I discovered an other libresoftware : uv3dp
This allows to convert SL1 files to CTB ones with a simple command :
uv3dp myfile.sl1 myfile.ctb
And just like that, you can use prusa-slicer with your mars2, without Chitubox or Lychee.
There is of course a bit of configuration to do in prusa-slicer : configuring the plate size, the resin properties, the printer, etc. I managed to do it taking parameters from Chitubox without encountering issues.
One thing Chitubox and Lychee do extremely well is manual support edition. In prusa-slicer, the features for manual support are extremely limited : you can only edit the contact point with the model. So moving a support pillar out of the way when it drops on your model is out of question, for example. That's a no deal if you're into manual support, like what 3dPrintingPro has taught us.
Lucky for me, I can use blender, so I do my manual support there, and it's actually way more powerful than anything Chitubox or Lychee allow : I can do fans like Lychee, I can give any specific support a custom size if I feel like it won't be strong enough, I can give contact points a custom size to minimize model's damage, I can go wild on the shape of the pillars, giving them as many articulations as I want, etc. It takes even more time than manual support in Chitubox/Lychee, but if you can use blender or a CAD software, you get the best result you possibly can.
Hope this helps.