Only have a G19.4, G42, and G21SF -- should I be on the lookout for 3 letters?
Sounds like a starter kit. Don't you know that you only really need one, maybe 2 glocks. To start get the Glock 20, a 10mm. Then, using mostly just barrels and mags, sometimes extractors and RSAs, you can end up with: .50 GI, .460 Rowland, .45 Super, .45 ACP, .38 Super, 9mm, .40 S&W and finally .357 Sig. You can do .22LR but that is a whole new slide and mags.
Your other option is the Glock 29 if you want to keep it small. I believe that you will lose several of the possible calibers by doing this. Although if you don't mind the barrel sticking out you can run the G20 barrels.
another option is to get a Glock in .40. With this you have the option of .40, 9mm and .357 Sig. Or, again, the .22LR full slide conversion.
Personal experience. I have a G21 and use it to shoot 10mm sometimes. I have a G23 and use it to shoot 9mm sometimes. I probably should have gotten the G20 instead of the G21 as the base platform, but there were no police trade ins with cool badges laser etched on the sides in 10mm.
On a side note. Supposedly you can shoot .40 out of a 10mm with no conversion of anything. The only way I have tried this was with my aftermarket (it has a fully supported chamber?) 10mm KKM barrel in my G21. This was hit and miss. I got stovepipes about every 3rd or 4th round. I haven't tinkered with different RSAs which may be causing the FTE issues, but they fire great.
As I say all of this, I'm not advising anyone to try any of it. I've had good luck and there are soooooooooooooooo many warnings about running different calibers in guns they were not originally meant for. So there is that.
One gun to rule them all...... Moooo-hoo-ha-ha-ha-haaaaaaaaaaaaaa