You're better off with a 40 gallon breeder or critter tank, as it's wider. The fish tanks are only 12" wide, not enough space for a grown dragon to turn around. The critter tanks are 18" wide so much more room. Plus being 18" tall, they're easier to heat. I have 5 dragons in critter tanks & they have lots of room to run around. My bigger dragon, Rubio, who's over 800g and 22" long, is in a 75 gallon, which is still 18" high but 20" deep & 4' long.
So having the wider tanks, no matter which type you want to get is better for your dragon.
100W should be sufficient to heat your dragon's basking spot because of the tank being 18" tall as opposed to 24" tall. Makes a difference.
I'm glad he's feeling better and eating well and being more active. How you see why we were all insistent on getting the temps right & having a good UVB tube.