I have couple of silent switch samples I wanted to compare one day so I took the time today to test them for you.
I've put them in some spare board to see what is the most quiet, keep in mind they're completely stock, only 1 switch sample rather than a full board, I've layed them on a top mounted plate in a row rather than trying them in the same spot, and they're differently spring weighted, which can also contribute to the pitch.
In terms of sound I'd rate them in the foillowing way (first one is the most quiet):
Boba Gum > Durock Dolphins > [Silent] Ink Black > Boba U4 > Cherry MX [Silent] Black > Cherry MX [Silent] Red RGB > Gateron [Silent] Yellow > Kailh BOX [Silent] Pink > Kailh BOX [Silent] Brown.
In terms of smoothness:
Boba Gum > Durock Dolphins > [Silent] Ink Black > Kailh BOX [Silent] Pink > Gateron [Silent] Yellow > Cherry MX [Silent] Black > Cherry MX [Silent] Red RGB > Boba U4 > Kailh BOX [Silent] Brown.
In terms of dampened feel (how soft the bottom out), first is the firmest:
Kailh BOX [Silent] Pink > Kailh BOX [Silent] Brown > [Silent] Ink Black > Gateron [Silent] Yellow > Cherry MX [Silent] Red RGB > Cherry MX [Silent] Black > Durock Dolphins > Boba Gum > Boba U4.
Bonus - Stem Wobble (first is the least wobble):
Boba U4 > Cherry MX [Silent] Black > Gateron [Silent] Yellow > Boba Gum > [Silent] Ink Black > Cherry MX [Silent] Red RGB > Durock Dolphins > Kailh BOX [Silent] Brown > Kailh BOX [Silent] Pink
Gazzew switches takes the crown once again, the Boba Gum excels in almost every category, it's very quiet quiet noticeably than the rest and very smooth, while it doesn't have the best stem wobble it's quiet good, though the only downside the bottom out is not the firmest and ranks quite a bit low, it's not bad but it's not the best, the switch doesn't have any spring crunch and you can basically use them stock right out of box/
Durock Dolphins are pretty nice too, they come pretty close to the Boba Gum, my sample have a slight spring crunch though.
Ink Blacks are pretty good too, very expensive though.
The U4 is great too but notice it's tactile, that's why it and the Kailh Brown ranked lower on the smoothness scale, it's smooth but it's not an important feature for tactile IMO, the Kailh BOX [Silent] Brown is not good, aside for it's lacking tactility, the tactile mechanism in Kailh BOX switches tends to develop a click sound after a while, my sample came with the click right away.
If you're gonna get one of the Cherry MX silent switches I'd recommend getting the Black housing version, the RGB one is significantly scratchier and have crazy spring crunch.
As for your other wonderings:
1. I'm unsure if you need to go all out for the GMMK Pro because it has dampening foam, a good keyboard with proper sound dampening can be pretty quiet too, the NK65EE has a pretty great sound dampening sillicone and you can purchase a 3rd party plate foam, alternatively you can get a cheap kit, sillicone made for molds and pour it into the case, basically minimizing echo from the case.
2. I'd recommend lubing the switches, it will obviously make them smoother, since for some reason the silent counterparts of popular switches tends to be less smooth, and it will also make the pitch lower, especially when using 205g0.
3. I'd highly recommended getting proper stabs, you don't have to get some fancy V2 stabs of popular brands, Everglide/Durock V1 are just as good, just mod them properly and don't stick some tape under the stabilizer housing (it causes it to bend) and you won't have any wire popping.
4. I'm not too knowledgeable about keycaps but from what I gathered, thicker is lower pithced, and ABS is bassier than PBT.
5. Double shot is not a concern for sound, you can read about the difference in various places that will explain better than me.
6. Do you even need O-rings and QMX clips? seems like a waste of money and alteringthe switch characteristics if you're getting a silenct switch already.