I haven’t had Shimano on a bike for years so can’t comment on heir quality / shifting – although a mate has Shimano on 3 different bikes which he does a lot of mileage on and hasn’t had much in the way of issues (except maybe 1 set of Shimano discs brakes failing with seal problems).
I’ve got Sram Rival on my racer, X9 / GX 10 speed on the hardtail and GX 11 speed on the FS. All work great with no issues.
Brake wise I’ve got deore discs on the hardtail and Guide r on the FS. The Guides blow deore into the water for power and modulation – but they are a more expensive brake so I guess they should.
Sounds like you should change your cable / cable outer for a decent quality one as your first move and hope that frees things up a bit. Although my hardtail has Clark’s inner cables and some ancient / mucky outers and the X9 / GX combo is working fine despite this.
Is your b screw adjusted right if you have ok shifts through some of the cassette and not the rest?