BF (23M) has Instagram with ex

My (23F) boyfriend (27M) and his ex (28F) dated for about two years; she broke up with him. This was in their senior year of uni. It ended very badly (he says she was manipulative), but they have reconciled about a year before he met me and they have remained friendly. He’s not a social media guy, but his last post is of his college graduation, just of him, but in the comments he profusely thanks her for being such a light in his life and helping him finish uni stronger and better, “the best partner he could have”. Doesn’t sound very toxic to me. We’ve argued before because she occasionally will send a message that I think crosses the line like “want to catch up sometime” and will be disappointed when he tells her we are still together. I asked him to delete the caption several times as I’m not on his Instagram at all and it’s the first picture anyone on his page sees. But I also understand his side. She was a big part of his life that he doesn’t want to erase and that I should trust him that he loves me and wants me. We’ve been together for almost a year and it’s been mostly very happy and secure. Should I just let it go? I cannot help but be angry when I think about the fact that I told him it makes me feel awful and he says that it’s really no big deal. I am sick of arguing with him about this woman though.

Tl;dr my boyfriend won’t delete an Instagram caption gushing about how wonderful his ex is and it makes me feel shitty. Am I overreacting and should I let it go?
Edit: age is wrong in the title, I’m 23 not him!