if Linux/Mac/Windows all have the same deadkey combination for ë and ê on their built-in AZERTY, use a macro that types that.
They do actually. Macro it is then.
For the ligatures, I might actually map AltGr-O and AltGr-A to some keys and Alt+0156 and Alt+0230 to adjacent keys, so there is no need to maintain layer spaghetti just for two (useful but relatively uncommon) characters.
A whole new can of worms can be opened too with accented, uppercase letters.
I’m gonna scale my ambitions down. Let’s not be more royalist than the king as we say in French, I will aim for parity with the AZERTY keyboard and we’ll see from there.