I only just started using 21700s. The 21700 is a little smaller in capacity than the 3x650 trash can ones. The good thing about those is they use less space than a pair of 18650s. The bad part is they charge at the speed of a single 18650 so they effectively take twice as long to charge. With a double 18650 I could get away with 4 batteries because by the time I’d used up the pair in my mod the others were charged. I had to use 3 3x650s because I’d have one on deck and ready, one still charging, and the empty in my mod. I seem to be usually able to get away with 2 21700s though ironically because they’re smaller than the trash can ones. Longer though. One of the nice things about the Tracy can ones is because both the 18650 and the 3x650 were the same size so I could use an 18650 in the 3x650 in a pinch. Won’t work with 21700s. Need an elongated cap or something. They may have even made one.Maybe that Geekvape you mentioned was the Solo. I have quite a few of them. Pretty bulletproof mods. Very comfortable to hold, IMO. Never had anything with bigger batteries. Since I do low power vaping an 18650 lasts plenty long enough for me.
Oddest thing I ever saw was an attachment you could get for a Silver Bullet mech. It replaced the top section of the mod and it had two 510 sockets on top so you could install 2 tanks. Can't remember whether it had 2 fire buttons or a switch to select the tank. Never had one but I saw pictures. I do have a collection of Silver Bullets for when all my regs die.
One thing I wish I had in my stash for nostalgia reasons is the cigalike I started vaping with. It was called E-light Elite, made in Chicago. I vaped them for years until I got my first Ego, then I threw them all out.