Any question answered...

it's 50/34, 11-36 (don't judge me, it's windy and hilly up in Scotland!). X9 2.1 long cage mech.

assuming compatibility of indexing etc if using different groups those ratios should work fine together

If I shift from lowest gear up one nothing happens until I shift up a second one. If I shift from 3rd lowest to 2nd lowest nothing happens until I shift to the lowest. So it's not something that the barrell adjuster will fix.

sometimes cables that are not moving smoothly through cable housing (old - corroded or catching for whatever reason) can cause cable tension like issues -

equally if something is bent that would throw it off and not unusual for issues to be more apparent when changing from one sprocket than another - the fact that issue only arises when in big ring makes me wonder if its chainline issue maybe causing the problem...