I have a 110 gallon tank and the 20 gallon would have been more than adequate. I am assuming you have the same 110 gallon, 48"x18"x30"h (same as a 75 but taller). I now use a Trigger Sys 34 gallon Sump and I have enough room for a pump off and ATO stuck on and fully pumped over plus another inch.
Most important thing is don't mix and match materials. Use glass baffles on glass tanks and acrylic baffles on acrylic tanks.
Regarding glass, I get the glass cut at a shop so they can grind the edges smooth. With glass, any basic silicone will work as you don't need high strength for baffles - but no mold inhibitors! I prefer to have the baffles cut about 1/8" short in length so they're easier to work with.
For acrylic, you have to use weld-on solvent so the cuts need to be more exact.
For your water height, refer to your skimmer documentation and set it there...skimmer should control the water height.
You do not need a dedicated area for heaters. I have mine in the pump bay but could also/or put them in the skimmer bay.
My UV is on my tank return manifold so is not in the sump. If you do have an in sump model, or a dedicated pump for it, put it after the skimmer so you are sterilizing the "cleaner" water.
You did not mention refugium, but people WAY over think these. There placement and size has been debated in the hobby for 2 decades now, if not longer. It will be fine no matter what you do, if you even use one.
For under baffles, you only need 1". For the top baffles, I prefer to go full height so the sump does not "short-cycle" when overfilled. For spacing between under-over baffles, I like 2". 1" is enough but 2" is easier to work with.
For your pump bay, make sure you have enough area that the height does not change drastically over the course of a day due to evaporation.
And remember to verify you have enough empty space to capture all potential water if your pump shut off. If you have check valves, assume they will fail, check valves will always fail at some point.
