Well, I at least expected to have different blue screen error with card reader. And no, it's actually my first time hard modding, so I'm just going through all card readers I have avaliable to see if it'll work.Not all card reader will work, have you ever done a hardmod before using that card reader you are using now?
I'll suggest for now is try another card reader, failing that it could be possibility be the soldering. Bad joints or wrong wires soldered to wrong test points.etc
Do you have a multi-meter with continuity?
Soldering I've done you can check in this post - https://gbatemp.net/threads/tutoria...-xl-n3ds-n3ds-xl.414498/page-113#post-7449203 There was already advice about bad CMD and CLK soldering, so I might try to redo all of it and use slightly less solder overall and see if it'll change anything.
Also, yes, I do have multi-tester with continuity. Any advices on what I should try and check with it?
Also, on matter of MCU - I still can't say if it's bricked, but visually MCU chip looks fine. Not burned out, at least.