Instead of getting a bigger tank, why not get another tank the same? Then, you can develop more in a session without having to dry out your tank and reels for another go.
Mix up some chemicals, develop the first tank-full (pouring the chemicals back into the jugs instead of down the sink), then repeat with the second tank!
I actually have three Paterson tanks (a 3 x 135, a 2 x 135 and a 1 x 135). I make enough chemicals to fill the largest one, do that one first and work my way down in size. It also has the advantage of allowing different developing times/agitations for different types of film, or different shooting conditions, without forcing everything into the same tank... oh, and you don't need so much volume of chemistry, either
Win-win-win!
Just this weekend, I developed 6 rolls of 135-36. The oldest was hanging around for about a year, but the others were much more recent. In my tanks, I had:
3 x HP5+
2 x Bergger Panchro 400
1 x HP5+
The nice thing was that I could give different developing time to the different films and, if I made a c0ck-up (like last time, when the "light-proof" funnel in the middle-sized tank fell off 3 times during developing!), then not all films would be affected. As it happened, no mistakes this time, and 6 very nicely-developed films to be had. Oh, and incidentally, the film from a year ago (just hanging around in my kitchen cupboard) looked identical to the one shot last week. No change in process required.