How to merge two KiCad 7.x PCB circuits into one

Schematics are easily copied/pasted between KiCad instances. I've done it multiple times. Just be aware that Edit -> Paste Special gives you some control over how to handle the reference designators.

So, if you give your "modules" very unique reference designators, then you could copy/paste the schematic with "Keep existing" followed by a copy/paste of the PCB section.

You would need to then re-link the two together. That is done with the Tools -> Update PCB from Schematic (also the F8 hotkey).

This would be one of the very few cases where you'd want to use the "Re-link footprints..." option. (otherwise, that should not be checked!) As the name implies, it'll re-link elements in the schematic and PCB using the reference designator instead of an internal id that KiCad assigns.

To be upfront, this is all theory on my side. While I regularly copy/paste Schematic blocks (letting KiCad auto-assign new designators), I haven't tried to do the same with re-usable blocks or PCB elements.

while I do copy regularly on the schematic side, apart form a few cases I usually do the pcb level layout routing fresh each time.

Same. It always seems like there is some constraint that makes me need to re-do it anyway.

I guess it would be different if I didn't care about the size of the PCBs.

my main constraint is that with every time I do I get better at routing/placing that design. so each time I reuse a circuit it also gets a bit better layed out.
the gains might diminish over time....

I confess I have discovered myself the design improvements that come from a wash-rinse-repeat cycle.

I get some satisfaction from working the schematic but find the PCB layout arduous.

I am the opposite!

I detest the schematic side but enjoy the puzzle and challenge of making a good layout.

I am the opposite!

I detest the schematic side but enjoy the puzzle and challenge of making a good layout.