Two air handlers, one furnace, and an odd control system. What's the modern equivalent?

I have 2 air handlers, each has their own AC, and each of them have a hydronic heat coil that leads to one furnace. Air conditioning works well independently, but there’s some weird controls when it gets to heating both of them.

There’s a board labeled WA200 that all the thermostats and air handlers are wired to. I found one on ebay, they call it "Advance System WA200 Water To Air 2 Zone Control Board Hydroair Circulator Panel." There were diagrams inside the box, but the previous electrician crossed them off and wrote "NOT APPLICABLE." It appears to have some features that turn on both the heating zone valve and the circulator pump when there’s a thermostat call for heat. This magic box also attempts to control fan speed to run a different speed for heat vs cool. The old thermostat was wired to the using a B wire WA200 as if it was a heat pump. I believe the use of the B wire may have been to change the fan speed in heat mode? There’s an aquastat on each hydronic coil so that the WA200 won’t blow air until the coil is hot, but those were bypassed by a previous tech for a currently unknown reason.

I recently had one AC and air handler replaced, and the tech did some odd workarounds to get it working. He prefered to do all his wiring at the air handler rather than in the WA200 box as it was done previously. I don’t really trust his wiring, he half-bypassed the box because he couldn’t figure out how it worked and destroyed a contactor in the process. I’d like to figure out how this control system works on my own so I can either fix it or advise him on how to fix it. He does have to come back to finish up some other cleanup items soon.

I feel like the modern approach to this would be using the thermostat (Nest) to control the fan and putting a zone controller on the furnace. We can have call for heat talk to the zone controller, the nest can control the fan speed. I’m not sure how the hydronic heat aquastat comes into play in this scenario, but as I said, previous tech bypassed it anyway. Maybe that aquastat goes in series with the call for heat?

What makes sense here? How can I stop the butchering of these poor HVAC controls? Should I retain the equipment that’s in place or do a full control swap out?