Apologies as I have not read the entire thread.
My experience of being in the position of wanting to update my title on my accounts without updating my name was similar to yours: mixed.
Some financial organisations, whether banks or others, had their system set up such that a change of title takes place via the same process as a change of name, and requires proof, the same as a change of name does.
Other organisations require no proof at all, and made the change over the phone.
At least one, when I submitted the paperwork to update my title, removed my title entirely from the account leaving me title-less. . I let that one go.
In case it makes any difference, there is at least one utility bill on which my title reflects the opposite gender to mine and I cannot remember whether I ever got around to getting that amended or not, and it has yet to cause an issue with my utility supply.
Personally, I decided I had bigger fish to fry and just left it, so I am my former title on some accounts and my new title on other accounts and the wrong title entirely on others.
In conclusion, YANBU to address the issue with your bank, but YABU to press the poor guy on the phone who is following the process set out by his bank into bending the rules that he is following. He is only doing his job, after all. Maybe just respond knowingly, "Ah, your bank is one of the ones whose database was built under patriarchy"?