Like @jay I went down a similar path. Designed the PCB to fit in the latest case, US type with 24VAC power, 6 wires R/C/G/W/Y/W2 & 4 relays for W/Y/G/W2. I added a pad as I wasn’t sure which pin W2 should connect to on the RPi, so it’ll have to be jumpered. I don’t plan to use it though.
Just received the boards, still waiting for some components.
Power supply is a simple bridge rectifier with smoothing cap for 24VAC -> 24VDC, feeding a LM2596 based LM7805 drop-in replacement 5V voltage regulator to keep the heat down. I’ll build up the power supply circuit first then test it. Next I’ll build & test each relay circuit individually. Raspberry Pi will get added last. I put the connector for the BME280 where I can use a right angle connector to keep overall height low. I’m also powering the BME280 module from the 5V regulator as it has it’s on 3.3V reg.
I’m an electronics hobbyist, a good percentage of the PCBs I design function - hopefully no magic smoke gets released with this one.