You mentioned "one display" so the MCU running that can run the side powering it. As for having the other side isolated and yet controlled by the same MCU, that'll likely require high-speed optical circuitry (shifting out and latching DAC data along with getting ADC data back to the MCU on the other side). That may be a stretch for this beginners forum.
*Edit - Skip the separate 5v supply, since 7805's can do it fine (and plan for 12 or 24v to run beefy output relays too).
Q - Have you considered a slaved, display-less MCU to efficiently run the other side?
(You'd still need to opto-exchange command, display and relay data, but the throughput rate wouldn't be so high.