I did a bunch of research on SCHS for a customer recently. They're gangbusters for cooling, but heating gain is minimal. The real winter advantage is that they heat up the ground earlier in the season, which doesn't do much for aquaponics. When I have time I'm going to make a calculator for how much heat/cooling to expect given your design. Maybe over the winter I'll do it.
Geothermal for water heat would be an interesting idea - haven't seen it before. Beware of geothermal sellers over-promising. The way efficiency for them is rated (COP) doesn't include pumping power, which can be a big part of their load. They're great, but not as great as they've been made to seem by some.
Last year when cold weather hit I rushed and built an 8x10 GH. I was happy with the results but struggled to keep it warm. This year I am expanding to 14x21 but will be able to reuse the walls. I am anticipating no further growth but will be building in a way that the GH can be expanded if needed. No deep digging to undo if there are changes of any kind. I have a sunken sump now and that will be the hardest item to relocate.
I'm kind of thinking like Sonja where I plan to do it once. The structure I am planning will be too big to dig it all up a second time.
That sounds great! Love to see pictures as it comes along!