I’ve been tempted to try this. There are lots of RP2040 based boards very load profile, which is what would probably try. Using USB for connectivity, rather than bluetooth. But its just theoretical, as havent tried it yet. In particular havent got round to trying to get meshtastic running on such devices. Likely to need a custom build of the firmware.
Sorry, that doesn’t actually help you!
Only other thought, maybe a custom broad. Cheapest node so far - #42 by Nestpebble - could be made very low profile. I think all components on one side, and through hole components could be avoided.
Includes an integrated lora antenna.