Thanks for sharing! It sounds like you’re getting good results with the filter regardless, but I’d be interested to see the numbers whenever you have a chance to measure again. I’ve been wanting to try something like this myself, but there are so many bad reviews for different BPFs out there, it’s hard to know which vendors to trust.
Did you roll your own BPF, or purchase it? If the latter which one did you get?
Kaihl choc switches (the V1 type) are going to have the shortest travel out of the keyboard switches that are common enough to use, and that’s what most low-profile hobbyist mechanical boards out there use. There’s a “choc amber” version that recently became available that have the shortest travel distance among choc switches (1mm actuation, 3mm overall), but they’re pricey and I haven’t seen anyone fill a keyboard with them yet. Those are also designed for fight pads, so I don’t think they’re made with silence in mind.
There are “silent” choc switches on their way (developed by Darryl lowprokb.ca), but they’re not available quite yet.
There’s also the Kaihl X switch with a 2mm overall travel distance, but they’re also pricey and I think it next to impossible to get key caps for them (at least it was last time I checked). Hazel is the only person I’ve seen make a keyboard for those: https://github.com/hazels-garage/shortstack. Awesome stuff, but probably not easily obtainable.
The Chouchou board looks like it is only compatible with choc switches, so that’s what you’ll need to get. In this case, “choc” is shorthand for “Kailh Choc (or Chocolate) V1” which is arguably the most popular low-profile switch in the mechanical keyboard community. There is a choc V2 switch as well, but few hobbyist boards are designed for it, and it’s not strictly compatible with the choc V2 PCB footprint. Do not buy choc V2 for the Chouchou.
Choc v1 switches require their own type of keycaps and are not compatible with the vast majority of keycaps which are designed for MX switches (which is the de facto “normal” switch you find on most mechanical keyboards that are not low-profile). You are correct that “1.5u” means that the keycap is 1.5 units long; you can use 1.5u keycaps for the thumb keys on Chouchou, but 1u keycaps will work too as you can see in the photo in their GitHub repo.
If building this board will be your first soldering project, then you’ve picked a good one to start with since you won’t have to worry about soldering tiny diodes or anything like that. This board has the option of using “hot swap” sockets for the choc switches if you think you’ll want to try different variations of the switches, but the PCB also allows you to solder the switches directly to the board if you want something more permanent (and fewer parts).
There is plenty more to know about mechanical keyboards, but the community is friendly, so ask your questions when they come up. Hope that helps!