Your disk problem could be anything from actually bad disks (they are bit rotting a lot nowadays) to a bad disk drive. If some disks are read OK I would lean towards thinking all the disks that fail are actually faulty. This is perfectly "normal", 3.5" double sided disks seem to be on the peak of bit rotting.
The only way to know if the disks are actually fine is trying them on the Amiga of someone who knows 100% their disk drive is 100% working. Also try to clean your disk drive with a disk drive cleaning kit, and if a disk makes a "whistling" sound, throw it away.
I can't comment on the PCMCIA issue, I am not totally sure about it, but to upgrade thr ROM you have to change the kickstart ROM chip inside the Amiga. it's a very easy operation, really. as you are competent and already figured out how to do it yourself, just take the usual care when dealing with electronic components (like grounding yourself as to not kill the chip with static).