A new standard PS4 controller would be $40-$60 here A new 20th Anniversary edition is $96 on amazon here You could swap a standard controller’s board into your 20th shell, but I do not know how different they might be
There is a detailed guide posted on reddit that you can try https://www.reddit.com/r/3DS/comments/5q... But officially on nintendo’s website it states you will need to buy a replacement SD Card from them https://en-americas-support.nintendo.com/app/answers/detail/...-card-or-microsd-card-damaged-or-lost Try the reddit link
Are you comfortable enough to open it up again? If so check all the connections to the logic board, make sure nothing is loose or torn on the right side. You could take it to Apple for repair but they’d likely notice the battery and charge you for any repairs.
Have you tried formatting the sd card without using the adapter? (IE plug card directly into your computer) The iFlash Solo might be bad or the ribbon cable connecting it could be slightly torn. Try contacting the manufacturer at https://www.iflash.xyz/contact/ if reseating the card doesn’t help.
You can jailbreak it with Seas0npass, that's about it. https://support.firecore.com/hc/en-us/articles/215090347-Jailbreaking-101-Seas0nPass It is pretty much an iPhone internally, everything is soldered to the board.
Honestly this is just an educated guess, but the memory might be failing. Try removing 1 module (if you have 2, if not try to find a replacement to test with) boot and test, if the problem persists swap the module and boot again. Let us know if this works.