Hands Down on 
My personal ZMK instance of Hands Down on small portable boards.
╭─────────────────────╮ ╭──────────────────────╮
ESC │ X W M G J │ │ #$ .: /* "[ '] │ BSP
TAB │ S C N T K | | ,; A E I H │ Tg(Num)
LSFT │ B P L D V │ │ -+ U O Y F │ RSFT
╰──────╮ App BSP R │ │ SPC RET TAB ╭──────╯
╰──────────────╯ ╰───────────────╯
I only have definitions for Vibranium (a.k.a Neu-vb), which is what I'm using at the moment. Vibranium is designed to work with all the other smart behaviors. Currently I have most of the essential combos (Qu,Z, Undo, Cut, Copy, Paste, etc.), and H digraph combos to send TH,CH,WH,SH,GH,PH bigrams. There's a ZMK take on Adaptive Keys (using macros and sticky layers) and Linger Keys (using macros and hold-tap behaviors for things like diacritics éèôåō and paired symbols, {([<>])} and more). They behave a bit differently than in QMK, but are quite usable. Eventually I hope to deploy a properly coded solution to these features.
Templates for Neu and Gold are in the works.
Currently building only for a Mac (hard coded for gui instead of ctrl, and some Mac specific keycodes), because I use this mainly with my iPads and phone when I'm on the road. My Semantic Keys feature for platform independence works on my QMK implementations, but isn't yet fully fleshed out and will require more invasive work for ZMK, so this too is for later.
Below is a much simplified view of my zen keymap (no adaptive or config layers, many combos not shown).
Thanks to jcmkk3 for the idea to implement adaptives via sticky layers, and many others on the Hands Down Discord for the endless creativity and support. Of course, huge thanks to Pete Johanson and contributors for ZMK, and for the exquisite Zaphod board design, and to Darryldh (lowprokb) for the sublime Corne-ish Zen board. Also thanks to caksoylar/Bravekarma on the ZMK Discord for the great keymap visualization tool, keymap drawer used to make the keymap images above, and a lot of help with some implementations.