I’ve been looking for a nice Bluetooth 60% that’s a) available from stock and b) running open source firmware for quite some time now and recently stumbled upon the “Common Keyboard Platform” (CKP) by Polarity Works.
For some reason those boards are seemingly never called “CKP” outside of that product page but referred to only as the BT60, BT65, BT75.
The ZMK firmware is very, very well documented and building it using GItHub Actions is a breeze: Fork the zmk-config-ckp repo, clone it to your machine, switch to the branch that fits your keyboard size, edit the keymap, git add, git commit, git push and download the resulting binary from the GitHub webpage. No toolchains, no pain, just unconditional trust in the cloud and a text editor. 
The Bluetooth Low Energy (BLE) module works really well and the way it’s handled by ZMK is also very convenient.
A half-arsed approach to a case compatibility list can be found here on Google Docs.
So I built the 60% version of these last weekend inside a spare aluminium Tofu60 case and I love it. The BLE connection just works perfectly. Night and day compared to the cheap GK6X it replaced.
I’m not using the underglow or backlight since neither my case nor the keycaps are translucent but it appears that lighting support in ZMK is somewhat rudimentary compared to QMK. So be aware of that if you’re in it for teh blinkenlights.