I would think that standardizing on Asuswrt-Merlin across all nodes would ensure similar/compatible Asuswrt components, if this matters. I believe AiMesh is closed-source within Asuswrt-Merlin.
Since using a mesh setup several months ago, I've kept both the AC86U router and AC68U node on the same Merlin versions, so currently both are on 348.19, without any issues.
Whatever you choose, it should be the same firmware 'flavor' for both.
RMerlin firmware is still the one I would suggest though. More performance, fixes, options (even if you don't use them today), and better stability too.