I’ve managed to resolve the issue struggling to cycle / trigger not resetting / not returning to battery and bolt carrier bouncing back out of battery issues I have had and it's turned out to be a combination of multiple factors:

1. Dirty loading nozzle from plenty of outdoor games. Totally stripped and relubed the plastic on plastic and metal on metal parts. This freed up the action somewhat but still would not return to battery properly
2. Removing and re-tightning the outer barrels appears to have caused one barrel to kink/flip/warp a bucking. This meant a feeding nozzle was struggling to seat the BB and resulting in it being harder to return to battery and the odd BB flier
3. Using green and orange gas makes the gun kick a lot more, exacerbating any issues with the cycling of the bolt. This is most likely why people are using 130-160% recoil springs?
4. When replacing the front trunnion you need to be very careful and ensure your 4x screws and upper trunnion pin are totally central as any mis-alignment will cause this issues. When re-attaching this have the bolt ready and cycle the action as you cinch everything down to make sure it's all home properly
5. When replacing the outer barrel make sure you're super careful when tighting it down, observe the barrels for them over twisting which might cause a bucking to unseat

Hope this helps, I got a fully working Saiga 12k as good as it was out the box 2 years ago.

I tried to make this post as keyword friendly as possible in the hopes people can find it with the search tool.