That sounds confusing and frustrating and I can entirely see how the difference in how the names/rules are applied could smack of racism or at least preferential treatment of native Japanese. I’ve decided since reading all these suggestions and a few articles that I will introduce myself to Japanese natives by my surname so that I am addressed the same way as they are addressed. This seems the most equal to me and infers equal respect. Of course if we are speaking in English then I would feel more compelled to use what is more suited to the situation in English (which can even vary then… as some situations are still formal enough to go by a title and surname rather than just first name).
I feel for you on the name situation. If you wanted to change to using your surname now it could be difficult to get everyone on the same page or to understand why. I have a similar problem of my own doing in my personal life. I stopped going by my first name in my personal life about 6 years ago, opting for my middle name instead, as I liked it and just wanted a sort of fresh start at that time and was meeting almost entirely new friends. The exception was that I didn’t expect my family to change what they’ve always known me as so they still call me by my first name and I kept my first name for all things business and work related. I got into a relationship since then so my boyfriend only knows me as my middle name and finds it weird when I’m called by my first name. His entire family knows me by my middle name, and now our families are on totally different pages with what they call me, so I feel like if/when they meet it is going to get real confusing. These days I decided that I would rather just stick with my original first name after all but I don’t actually care enough to stop others who only knew me by my middle name from calling me by it, so I created a confusing monster and the only way to undo it would be to firmly and definitively decide one name as the only appropriate option and tell everyone that’s the one, which… as capricious as I am sometimes, I don’t want to do. Haha.
So as not to screw myself that way in Japanese too, I’m just going to strictly follow their cultural conventions when speaking in Japanese because that makes the most sense to me and would make me feel I am on “the same level” and not looked down on or something. Even as a non-Japanese, (given-name)-san sounds weirdly “cutesy” to me, almost like calling me Mr. (given name).