You should be able to buy a threaded 10mm barrel from Lone Wolf or KKM.
A .40 caliber suppressor would work as long as the manufacturer has rated it for 10mm. That's something you'd have to check with them about.
For subsonic loads, check any published reloading reference. Work up a load that is 1100 fps or slower. No magic to it.
If you're going to shoot subsonic ammo only, you wouldn't lose much by going to a .40, other that you could shoot heavier bullets with the 10.
My other recommendation is that if you're already looking at a Glock 20 with a 6" barrel, another 6" of suppressor on the end, and a red dot sight, you might as well look at an SBR on the AR-15 platform chambered in something like .300 Fireball/Whisper/Blackout. You'll be able to shoot heavier bullets (240gr .30 caliber) and it will be a lot easier to make hits with as a "long gun"; however, it won't be that much larger than your Glock 20 with all the stuff on it.