Try dedicated silent switches, like Cherry MX Silent red or Silent black, they have integrated dampening pads on the switch stem, 10000x more effective than o rings. I would suggest the Leopold 750r/900r/980r as they have high quality 1.5mm PBT Doubleshot caps and a dampening felt layer beneath the pcb and comes with the option for silent switches. Imo it's the best pre built keyboard and probabily the most silent mx prebuilt.
You can also take a look at the Silence Topre realforce keyboards they're really really quiet, except the stablizers which are quite loud but you can remedy that with some lube.
There's a lot more options if you decide to go the custom route as well.