I have been using sumps for over a decade on 100+ gallon tanks. When it was >3x tank volume it was quiet enough with a single drain but now I have ~6-8x the double Herbie type drain is silent and I only hear the pumps. Hoping once the doors are on much of that will be gone and the more I fuss with stuffing foam between pipes/tubes and what they touch the better it gets as well.
There shouldn't be much more CO2 loss with a sump with a calm water surface as with a canister. My skimmer usually drops water an inch or less so very little turbulence there either.
You forgot how nice it is to have a constant water level in the display tank!
Now that I have an old tank for a sump instead of a rubbermaid tub I can even put the thermometer into the sump if I like. Drop checkers could go as well but since they are for noticing that CO2 isn't going in probably wouldn't be a good idea as too out of the way for quick checking. If the overflow box is where you check it regularly and there is room the DC and thermometer could go it there.