Here's some suggestions that may help, each situation is unique it seems, so it's hard to recommend much.
A hot tub is a nice load; small, low temperature, and steady, maybe 1000 to 3000 btu per hour, which is like a half a gpm with a 10 degree deltaT.
It looks like you have a small amount of space above your tanks up in the joists. You could tuck a small plate HX or sidearm up in there, or if you are lucky there may be a place under a cabinet or in a closet on the floor above. Then you could use gravity to move hot water from the top of storage to the HX and let the cooled water fall back to the bottom of storage, which eliminates one pump.
Then just run a bronze or SS pump through 1/2" pex over and back to the hot tub. I 'arm' mine with a timer for one hour out of three and use a PID controller to cycle the pump on and off so it can sense the tub temperature and slowly add heat as necessary while it is armed.
Permanently disconnecting the electric heating elements was very satisfying, as was running only 12 ga. wire for just the pumps instead of 6 ga. or whatever for the whole deal.
I just teed into one of the the jet pipes to pull to the HX, and then teed into the drain pipe for the return. You may need a zone valve if running the hot tub would pump water through the HX, or if there is even the smallest amount of elevation that could cause thermo siphon heating.
[Incidentally I ran my hot tub heating circuit as a primary secondary, with the primary circulating from the tub through a UV sterilizer and back to the tub, while the secondary injects heated water as necessary. We adjust for pH and hardness and that's about it.]