Yep, I went and screwed that pooch. Got all my parts from mcmaster, put it together with teflon tape assuming that if it leaked, I could just undo it and use better silicone sealant. Ran it on plain water to test (and cooked a sous vide dinner at the same time, heh), and each joint leaked just a little bit. Ended up tightening them down a bit more, but still leaky. So I went to disassemble it in preparation for the better sealant.
All six of the 1.5" joins are stuck firm (the pipe nipple to the two tees, plus the 1.5" x 0.75" bushing on the other parts of the tees). I managed to back out the 0.75" bushings that I was using to reduce down to the QDs and RTD ports, but that's it. Also got out the heater element.
Going to spend the day working with penetrating oil and see if that helps, but at this point my choices are: invest in a big 2" wrench and hope, or switch over to a different technique. I think I might try a copper/sweat solution after all. If I'm clever, I can probably get that working relatively cheaply.
My other option is to put the whole thing back together and maybe use some kind of silicone caulk to manually seal the leaky parts of the thread, but that's unsatisfying.
There's a neat picture
I found online of a dude who put a flange on a piece of copper pipe (basically soldering a bit of 6-gauge wire all around it), so I will try to do the same thing- cut the tube in half and add the flange / triclover joint to make cleaning easier. Pricing it out makes it still seem a lot cheaper than stainless (even cheaper than aluminum, but you've got to actually know how to sweat the pipe yourself).