A few questions?

I have gone most of my salt water history without a sump. A sump is nice because it gives you a place to put all of your equipment (skimmer, pump, heater, carbon, refugium). From my perspective that is the main purpose. For me having a sump for simply that reason has been way better than not having one.

As for adding water volume it depends on the size of sump and how you use it. If you are planning on joining the water from your 5.5 nano and your 20 gallon tank into the same sump this would make your 5.5 nano much more stable. If you are increasing your total water volume from 20 to 30 gallons then it will help but I don't think it will be a big difference.

I would not be worried about a tank recycle by simply adding a sump. If you added a sump and a bunch of new live rock to that sump then I would be worried.

The one advantage I could see about a canister filter is a place to add carbon or other filtration media. I would worry that it wouldn't be maintained all that well and become a nitrate factory.

Sumps are great. I love having them. Your biological filter is located on surfaces like your rock and sand not in the water column. You can add as much water as you want (as long as it is similar in temp/ph/salinity etc without restarting the cycle. If you are moving the tank you will need to keep your rock wet. You can just get some buckets or tubs to transport the rock. That will prevent the bacteria on the rock from dying. If you are going a longer distance then you will need to keep it at the right temp. make sure you have the new salt water pre mixed and up to temperature before you add it to the tank.