I can't do AIWB - it seems I'm always carrying something or leaning over a fender. I'm the same height and this might not help, but not all "long" t-shirts are created equal. Carhartt and True Classic are LONG and worth a try. I prefer the True Classic as they fit decent without the square sail effect some t-shirts get in the larger sizes. Size up though, they run a little small. I'd guess you for an XLT. If you wait for a sale and buy a pack of six they are also cheap. True Classic is by no means a heavy t-shirt like some, but I prefer that for summer.
There are plenty of smaller pistols that offer similar round count to the G19. Proper size pistol, good holster and belt, long t-shirt - done. I carry a P365 OWB (Bianchi Illusion) on an Anson CC ratchet belt under a long t-shirt with no issue.
