Thanks for the tips, people! I’m loving this community!
Here’s a diagram for what I currently have:
And here’s what I’m proposing in order to gain maximum functionality using only existing wiring:
In my proposed solution, Sonoff 1 will be used solely as a relay for the actual mains. This can be controlled remotely using HA, app, or physically, using the button on the Sonoff itself. I may also add some sort of button to the GPIOs so I can have a small externally-available button to save opening up the light fitting if everything else is down.
Sonoff 2 would then not be used as a relay at all. Instead, I’ll use a rotary encoder (KY-040) wired into the GPIOs to allow changing the brightness by turning, soft on/off by short-pressing the button, and cycling through predefined colours by long-press.
I know I don’t really need a Sonoff in position 2 - I could use a D1 mini or similar - but I like the fact that the Sonoff is mains powered, and by the time I’ve sorted out power for one of the other boards, I may as well have just got a Sonoff anyway.
The theory of this solution makes sense to me, but feedback would be much appreciated!