I have build a set of seven macros which make the channel editing as well as conversion of Affinities "RGBA" to R-G-B-A,
RGBA to RGB-A as well as other tasks much easier and faster within Affinity Photo.
Big thanks to @NotMyFault and @,,, for their insight into Alpha channel math and how it is working inside the Affinity Suite.
The from scratch macros allow stacking transparent vectors or textlayers on top of each other.
This works as long as you keep the black fill layer for each channel intact and active.
https://myclay.gumroad.com/l/Affinity_channelEditmacros