Probably ride it out with what you've got and see how you get on - might be surprised what you'll get out of a 5 gal. I'd be inclined to just turn what you've got into a RDWC system with one big tote as a res (25-30 gal) on the next run. I grow 4 under 2 x 250w LEDs and lights seem to be the limiting factor (about 6-8 Oz per plant). They are in 4 gal pots with the res filled with 20 gal of soup. When they are drinking heavily in flower I top off the main res maybe 2-3 times a week. But it's way easier in an RDWC - just empty the dehumidifier back into the external res and drain/refill it with nutes once a week. Keep the res outside the grow space to control the temp better and make it easier to fill/drain.
I have 2 plants in my 5x5 with a 1000W light and I think I vegged em too long already lol they are huge. I thought about doing RDWC but DWC was lower bar to entry (they are pretty close since you can just buy a full RDWC system.
I like what you said about the water outside the tent but don't you have to run a pump to recirculate the water which is constantly increase the temp of the water because the pump will off put heat when it moves it around? This was the part of a RWDC I didn't really like.
Right now in veg the plant seems to drink the entire bucket in a day! I was thinking of getting one of these huge totes just for one plant, I've seen pics some where person doing similar with two plants in one and I can see how that would work since I'm doing 5g per plant now..
I have a four plant limit that's why everything is so scaled.