Just a little update on this!
After reading the thread here:
I took a chance on a Club3D CAC-1334 adapter (active HDMI →USB-C). And as I suspected, the active conversion sidesteps the weird incompatibility in the direct hdmi connection, and it looks like it works perfectly from the short test I’ve done! The display is battery-powered and will provide enough power through the USB-C connection for the active adapter.
In short: If anyone has had their eyes on the above-mentioned portable monitor, the hdmi connection doesn’t seem to work with the Pocket Reform (at least with the RK3588 module), but an active converter like the Club3D one may well solve it.