I'm with Steve on that one: it would be much easier to just pour your slab all the way out to the block on the inside. From the pic you posted, it looks like you went below grade along the footer with the xps board. The deeper underground you go with the foam, the harder it will be for heat to find its way in from below. In any case, I would definitely sheath the footer down below the frost line. Also remember not to make a moat around your footer: make sure it can drain and will not act as a wick that can hydrate your block.
I've been looking around on the web about air-to-water systems, and have found a couple of new things. They are much more popular in Eurasia than in America. In the last 5 to 10 years, the Chinese and Indian manufacturers have been conjuring up all kinds of inexpensive systems. A decent number of OEM's that sell complete systems also market "universal replacement control board" modules. These "brain in a box"packages can be used to replace defective controls in existing systems, or to modify or invent your own equipment. The offerings range from "standard window A/C controller" to "programmable commercial control box". Some of the air-to-water controls I happened across:
Qunda QD25a is commercial, QD26a is residential.
https://www.aliexpress.com/item/PCB-...c-4ce8d11a5ca4