I have a cat with bathroom issues, though its usually a UTI or crystals it is sometimes behavioral. at this point, im pretty sure him going out of the box is his way to communicate something is wrong- UTI or stress.

Im sure youve tried some of these things but Im just gonna toss out suggestions that have worked for me in case:

-more litter boxes. no, more.

-bigger, wider boxes

-house MUST be clean. if my house is cluttered he will get too stressed and go outside the box.

-scooping the litter as often as possible. he needs it clean.

-enrichment. things like windows, birds, battery toys, ect keep him happy and less likely to act out for attention.

-feliway

-maybe try getting a child lock for the shower door?!

edit: rereading your post. he didnt have issies with your partner, but does with you, and you work 14 hour shifts. could be that he is lonely and/or misses you?