I like the idea of using a temp comp valve, I could easily source one from work and may do so when I do the big tank (got a line on a 150 for a good price). Plumbing into the sump is a great idea for near zero input maintenance.
For my upstairs tanks where I have both tropical and coldwater I'm thinking of a hose bib fed by both hot and cold valves with a thermometer installed in the outlet to match tank temp. Along with a pump to start the suction for water changes, what is the venturi that you used to siphon? That may be easier and cheaper than an electric pump.
Gonna end up with two 55s, a 33 cube, 2 20 longs and a few 10s upstairs so I want an easy maintenance setup.
I will say your daughter should at least once enjoy the old way of doing water changes, it's a blast 🤣 that was the main reason I got rid of my 220 years ago.