Hello and welcome!
The most important thing in making a keyboard quiet is generally going to be switches, and all three you listed are good options. That said, they have different strengths and weaknesses.
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Silent Alpacas (and other JWKs): will require the most work, but will likely be the most smooth of the three once that’s done
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Silent Gaterons: probably the least quiet of the three, but personally I enjoy the sound and feel of these. Ink versions will be more smooth, milky & black versions will have more texture and light “swish” noise in comparison
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Silent Outemus: the most quiet, especially with lubing - but also the most mushy / least crisp
The case mods all help, too - but you’ll gain the most ground by starting with a quiet switch.
O-rings:
The ones for switches reduce travel and introduce mush; I don’t really recommend them. On the other hand, small o-rings to use as dampeners for tray mount cases or big o-rings to use as mounts for gasket cases are both excellent ways to further improve the acoustics.