I've always found that fitness is a cycle of cycles for me. Illness, injuries, periodic time crunches, and plain laziness have had me fluctuate in weight and fitness level quite a bit.
That said, good gear never changes. I think you might try investing in better holsters/belts before spending money on guns, which is a much more expensive proposition. One belt I've really come to love for AIWB is the
Meginjord by V Development Group. I've found this takes pressure off my pelvis and lets me conceal an AIWB pistol much ore easily.
I also very much like the
JMCK wing claw 2.0 (2.5 is also probably great) holster series and the
Seraph from VDevGru. Both make carrying a doublestack pistol substantially more easy.
If you do opt for a smaller gun over time, I'd go with a 43, just for the sake of commonality with your Glock 19 (assuming you aren't looking to move away from Glocks). The M&P Shield is also a nice gun and not so different from the G19; I carried one for a while and liked it quite a bit.