I finally connected all venting solenoids and added an oil separator and a muffler.

With, that there is no more oil on the bottom and the venting got a lot quieter.

And with that, I think the project is complete.

I already used the new compressor for plasma-cutting, cleaning, and powering my old industrial CIJ printer and it's working very well and it's quiet enough for using it at night without disturbing my neighbors/roommates.