I don’t think it is a ‘Flint’ topic, I do think it will be the same with every wireless device.
WLAN is not only about distance and wall thickness. It is also about the material of the wall. A 10cm concrete with metal net will be more absorbing than a 20cm wooden wall that is hiding some pipes. But I think this is clear.
And than there are the reflections. Reflections are bad. A reflection in the distance of the right wavelength is able to (nearly) neutralize the signal. Not very likely, since the Flint got more than one antenna. But I would try to move it 10cm to any side.

If your 2,4 band is not overcrowded you could switch from 5GHz to 2,4 GHz WLAN. 5GHz is newer and faster and better. But it comes with the law of physics: Higher frequency, higher loss. In some cases 2,4 GHz will be more suitable.

About the Temp: My Beryl (3 connected devices, WireGuard tunnel internal) also getting hand warm, like any other router I’m using. I would not worry about this.
Right now at my desktop:

Device Temp
GL.iNet Beryl 35,5°C
Netgear GS108 29,1°C
hp 24es 26,3°C
Lenovo Thinkpad x200s 31,1°C

I would be worried if it is lower.