Sorry to necro this thread a bit, but I saw your comment and wanted to recommend a site I recently found that’s exactly like PCPartPicker for keyboards: https://keyboardpartpicker.com/
I haven’t used it myself much, but it seems like exactly what you need.
Their website mentions that “both 3-pin and 5-pin MX mechanical switches” are supported. 3-pin switches are also known as “plate mount” switches and 5-pin are also known as “PCB mount”. I’d recommend getting a switch tester / sampler off Amazon to get an idea for what switches you want (linear vs tactile vs clicky). There’s a lot of switches out there and most of them will fit the keyboard because it supports 3 and 5-pin. I personally like Boba U4 silent tactile switches.
I’m not sure about the keycaps. Keychron’s website also mentioned that their RGB works with OEM or Cherry profile keycaps, but I’m not sure if they have a specific layout on the bottom row that you need to take into consideration.
That’s correct about the red / brown / blue.
For your keyboard there’s no major difference between 3-pin and 5-pin switches, but for other keyboards (or rather the PCBs within other keyboards) it does matter. You can use a 3-pin switch on a 5-pin PCB, but you can’t use a 5-pin switch on a 3-pin PCB (technically you can clip off 2 plastic pins on a 5-pin switch and it would work with a 3-pin PCB).
The 5-pin switches do offer extra stability versus 3-pin switches, but it’s not a big difference.
The most important part of picking a switch is deciding your favorite between linear vs tactile vs clicky, then narrowing down based on other factors like spring weight and mushiness. You can do that through trial and error or by looking up reviews / information about switches. I still recommend getting a switch tester / sampler, but you can also check out ThereminGoat’s reviews at: https://github.com/ThereminGoat/switch-scores.