The Reddit enthusiast consensus for maximum silence: pair a barebones board (like GMK101) with TTC Frozen V2 or Outemu Silent Lime switches. Owners report 40-45dB actuation — quieter than any prebuilt.
If you're willing to spend 30 minutes swapping switches, this DIY route gets you quieter results than anything you can buy off the shelf — the TTC Frozen V2 in particular is the community's current gold standard for silent linears.
“The quietest silent switches I tried are Outemu Ice Snow and TTC Frozen V2. Both are 40-45g linears, I replaced the springs with 60g ones.”