I don't know the specifics of your situation (aside from what you described). I will just throw out a few things that crossed my mind.
- I've found that over the course of life, especially with relationships that are more virtual, people will remain for a season and then the tie loosens up a bit. There are people I used to talk to daily (online) ten years ago who I have not heard from in years now. There are people I am friends with now who I did not know ten years ago. I also have some friends who I met IRL who I was close to in my 20's and who I don't really talk much to right now, ,but when we do connect, we can leap back in where we left off. So the relationship is still there, just not being actively maintained. I used to take this more hard in the past, as if I was losing people. Now I have been trying to just accept it as a fact of existence. Of course it is the most optimistic view, there will always be some people you will lose touch with because of bad action on one or both people's parts, but anyway I'm just saying ebb and flow is normal in relationships in a global world.
- I am always careful with giving money to friends. I find it easier to give to people I'm only acquaintances with or not close to per se, because I think then you fall back on more explicit rules. Things get muddy with friends sometimes. I have always found it helpful to be explicit when you give money to a friend, rather than giving in an open-ended way -- basically being very clear on whether you expect the money to be returned or not, and what the expected time frame is. Then the other person can take it or leave it. Invariably if the terms are not clear, one person or both will end up with hurt feelings when their expectations are not met. For example, I once gave a friend money to pay his mortgage for a month because he was in dire straits. It didn't seem clear whether he could pay it back, and I told him up front he did not have to, if he could not afford to. That was many years ago and he never did pay me back (lol) but I had accepted that option up front so there were no hard feelings or unmet expectations. I gave to him knowing he probably could not pay me back.
- I think it is clear from your example that your friend will not often be initiating contact between the two of you, and so I guess you would have to accept that that is his new norm and decide how much you want to invest. I think sometimes we overcomplicate things because we don't want to let someone else down but also because we don't want to let go of something that is passing. It's clear his norm at the moment is twice a year. Your norm is more monthly. You can keep responding to him monthly but just should remember he might not respond and be okay with that.
- it seems clear he doesn't really plan to repay you at least in the near future -- basically a person who just "forgot" or has the money would have just repaid you, and/or apologized for not repaying you and given you a date by which he would repay you... yet your friend's response was just to go off-grid. This pretty clearly says he's either unable to pay and is embarrassed (so he can't pay you) and can't even discuss it, or he is resentful that you are asking to be repaid. In general, I would consider this a sunk cost at this point. It's good you stopped lending him money, especially if it was considered "lent" and not just spent.
I do see you suggest something about "fairness" in that he should at least respond to you due to your loyal friendship over the years. You are not incorrect. He should respond. But he isn't responding. And you have no way to make him respond, honestly. So where does this leave you? You have basically realized you have been playing under different rules than he has. So how will you accommodate this new information about your relationship?