One idea is to mount the extension board under the big heat sink!
There is a post about it:
(big heatsink + standoffs from Rock Pi acrylic case can be used for it)
I faced the same issue. Recently I found a solution thats for me, which is to erase bootloader on eMMC to disable boot from it. Can give it a try if interested:
Its possible (but tedious) to remove the eMMC without removing the heatsink. A small screwdriver or spoon can be used to pluck or set it!
If you have any doubts, post about it in the respective topics!
