I had similar problems with my first 29er (a hardtail). I’m about 86kg with full winter kit on, and tried some Pro2 + Crest wheels on there. Trail centre reds were mostly OK, but on rougher natural rides it felt like the rear axle was loose – it wasn’t, it was the rim flexing. Swapped to Arch EX’s with same hubs and spoke type, and that solved the problem.
If you want a decent quality stiff rim, I can recommend the latest 25 & 30mm internal width DT stuff – have some of the 30’s on my long travel 29 and they are brilliant. No flex, but not harsh with it either.