Additional Testosterone is converted into Estrogen because the body sees it as "maintaining the balance" and isn't directly related to body fat content. Estrogen is fat soluble which does mean that when you have additional body fat it can create a negative feedback loop in a sense, but again body fat isn't a reason to prescribe an AI or not.

I'm not sure where the AI causing or being related to infertility comes from. Taking normal Testosterone will cause a suppression of sperm production while on TRT but that is only as temporary as the treatment & can be easily offset by other medications if you want to stay on TRT and have children at the same time.

AIs themselves typically no negative bearing on fertility long or short term, and in fact there are studies out there showing the opposite.https://pubmed.ncbi.nlm.nih.gov/23103016/

I would follow the advice of whoever your provider is at the time, but I would also highly suggest you get a second opinion. Endocrinologists are not heavily trained in TRT for males unless they seek it out themselves, especially if they are older (and not to be sexist) or are a woman. I say that because we've heard horror stories of endocrinologists who have berated men and made them feel terrible about themselves, calling them users or less than a man, just because they didn't know better.

Be aware that all Doctors are people, the same kind of people who work in any other profession. We have worked with many in healthcare who seldom know everything about the body outside of where they decide to spend their time learning.

Main opinion: Get a second opinion from someone who specialized in men's health and wants you to be your best. You sound like a candidate for TRT to me. I (Brian) also started TRT in my 20s.