Ran the old 3-way fridge from AC today to see how it performs. No problem running it at the same time as the chest fridge; it draws around the same amount of watts.
That is with the spare probe moved to a cup of water in it, until the end when I took it out to measure the final air temp.
Unsurprisingly, resistive cooling is a lot less efficient. Compare with the initial cooldown of the chest fridge at the top of this thread, which went to zero in half an hour. The entire loaded chest fridge cooled down faster today than the empty 3-way. On the other hand, it is silent and I can cycle it on and off with every passing cloud. Seems worth automating that and seeing if I can use it as a little freezer.