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MTB Bum bags are a pretty good option if you want to keep weight off the bike, particularly ones with a bladder - frees up the frame for a full bag.
I've had a bad time on the mountainbike with a backpack before because of the swing side to side, but I have trail running pack that works great. It's a BD Distance 8 which fits like a limpet and doesn't affect handling. all of their mountain running packs like the Blitz crossover pretty well for for activities like climbing and mountainbiking where you don't want to compromise mobility.
Surprised that weight hanging off the arse didn't bother you.
I don't mind 1.5kg on the bars, if it's in a good holster. But weight off the arse is annoying. Sadly if riding proper MTB a backpack wins, but is annoying.