Solutions
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popcicle2003's post in Cache disk has become unmountable was marked as the answerOk, I created a backup of the cache on my main array. I then ran btrfs check --repair /dev/sdd1 and it seems to have worked™. We will see if it holds, but if not, yeah, I might have to format and recreate the pool. Thanks guys for the help! (Slso for anyone who might have a similar issue, it's not too late to make a backup of your cache before you mess with stuff)!!!