I improved my pp a lot by spending time on cambridge in colour where just about all use PS. A number of people on there are extremely capable. What I find is that no single open source package will match PS. The ones I use most often are Rawtherapee, Fotoxx and the GIMP Also HUGIN and Macrofusion Enfuse at times. Fotoxx is purely a Linux application. The maintainer sees it as a photographer’s swiss army knife. It’s a pretty good description of it. For raw I always start with RT.
What you will miss is adjustment layers. The GIMP doesn’t have them and it will take anyone some time to pick up all of the things it’s capable of.
Another package that did specialise in more exotic filters is Photivo. Development seems to have largely stopped.
It seems certain other filters can be used from Linux
I’ve had a bit of a photo lay off and that one is something I intend to look at in more detail.