The K-series, or even the V-series should be fine. To get quiet in an affordable board, you need:
-decent stabilizers that come pre-lubed
-dampening
A lot of the Keychron builds come with these things. Some Keychron keyboards come with PCB-mount stabilizers, hopefully pre-lubed. I can tell you from experience that pre-lubed PCB-mount stabilizers will make a significant difference in an affordable board being quiet.
So, basically Keychron is a good choice. Get something with PCB-mount stabilizers, if possible, and comes with dampening material [foam and stuff]. U4 Boba is also an excellent choice of switches, so wait for them to come in stock. [You don't really need to lube U4 Boba, even though it helps. It's really helpful to at least lube the springs, though, which can be done relatively quickly through bag-lubing.]
If I needed a quiet keyboard, I would buy a Keychron Q6 and stuff it with OUTEMU switches similar to U4 Boba. That alone would be 60-80% of an ideal build. Lubing would bring it the last 20-40%. Of course, a Q-series or even V-series would be fine. Plastic cases are probably easier to silence than metal, and you don't need fancy mounting options with silent switches.