Well the modem (5G/LTE) and CPU will be getting really hot, as it eventually runs 24/7.
The reason why routers are chunky beyond RJ45 height is often also the space needed for the heatsink, made from bent (W/Y shape) aluminium plate.
Adding to it WireGuard will be used much, that is heavy on CPU and it will boil itself quickly.
For this reason the battery will suffer - heating up as well (dis/charging), at least it should be as a separate box with an air gap between the router and the batteries.
Also WiFi 6 and later are RAM (afaik 256 MB RAM is normal now, better to have 512 with OpenWrt) and CPU hungry so there is that, adding to nonstop load.
So the design should really dissipate heat well and be able to connect the heatsink to the surface - as presumably a fan is to be avoided.
And the parts must be a snug for the thermal pads to keep on place in contact - because of moving it frequently put this under pressure.
