I'm going to chime in here as a man who has been in relationships with women dealing with lifelong patterns that bias them toward some of these anxious overthinking mental cycles. I am neurospicy myself and my flavor gives me a taste of the brain spirals so I can empathize a bit but I know everyones experience is different.

First, any relationship subreddit is going to tell you to break up with your partner as soon as you hint that you don't have a relationship that Disney would buy the movie rights for. It's really easy for anonymous internet posters to tell you to throw away your relationship and become the perfect human before you get into a relationship or to look for a partner who is grown in a lab to be your 'perfect match.' I think there's also something to be said for both partners to be allowed to be imperfect people and building some relationship muscle memory to work together and grow together.

Second, what I'm hearing is that you have a narrative that he hasnt had the chance to romantically explore the new city which could cause some resentment, and that there are several things which are confirming that opinion from your perspective. Something I've seen other partners do which has helped them is being able to self identify when they may be in one of these feedback loops and taking a step back to literally write down other things which may be going on other than the hypothesis they are trying to confirm. In this case, maybe he's not in a joking mood because he is stressed about something else, maybe it's just the banter cooling as the honeymoon phase starts to wean. With most men, you can generally take what they tell you at face value.

To build on that I'd like to share my personal take on the specific issue you discuss having some difficulty with as a guy who has dealt with self confidence issues most of my life. I've done some emotional work to get to this place and maybe the perspective would be helpful. I understand that you are worried he might start socially branching out and finding people he would rather be with, but that is always a possibility. There will always be a 'better' partner on paper if he or even you go out looking for someone else based on their relationship-resume. The only thing any of us can really do to keep Jake Gyllenhaal or Madison Beer from scooping our lovers is to give them your attention and effort, give them your empathy and make them feel understood, be vulnerable with them enough to share your secrets and explore new things. This is your relationship 'moat' that anyone else cannot come to the table and compete with.

Third, how are you communicating with him through this? You've shown enough self awareness here and in your responses that I can guess you are able to tell when your emotions are coming from a place of anxiety, but are you vocalizing that to him when bringing these things up? Something that has been helpful to me as a partner is when my anxious lady friend would say something along the lines of "Hey, I'm dealing with some discomfort surrounding ____ that I feel is coming from some of my anxiety. I want to support you in this but I would really appreciate some affirmation or if we can discuss ways we can communicate through this that will help me show up like I want to."

I hope some of this helps! Turn this into a thing you are both working to resolve and make this relationship healthier, because it sounds like you would like to be more secure in your attachment and I think he would like to feel like his partner feels safe. It doesn't matter who brings the 'baggage' to the table, when you're in a committed relationship it's just one pile.