I love oldschool FPS’s, but somehow the whole boomer shooter resurgence just passed me by. I just finished Boltgun on the hardest difficulty and had a great time, would love to hear some other suggestions
DUSK and CULTIC come to mind. The original 1993 DOOM and 1996 Quake are also absolutely still worth playing, but it sounds like you may have already played them. Maybe Post Void, Cruelty Squad, or HROT if those seem up your alley.
I’ve heard great stuff about ULTRAKILL, but I would wait for it to leave early access before getting it.