Thanks for the tryout and the report. Its power consumption was to be expected as the origin of the heat. It should be only plugged in when used immediately and removed right afterwards.
I plan to switch its casing with an official Framework USB-A expansion card to achieve the same result, without having anything sticking out, by applying the thermal pad to the other side of the expansion card and transferring the heat via the official expansion card’s metallic lid to the laptop chassis. Finally, inserting the USB-C-to-A adapter into the former card reader casing to use it as a dedicated adapter.
Btw, do you have any tips regarding the removal of the card reader out of its metallic casing without breaking it?