Six months is when the honeymoon's done, you are correct. You have a solid idea of what to expect from one another, sex has been explored, and the novelty's done. You also start seeing signs of lack of compatibility: conversation dwindles, friction points are clear and the shortcomings of the pairing become more evident because both people are back in their baseline mode.
It is also when your relationship actually begins, in my opinion.
Be careful when you try to "find the deeper meaning of his actions". Sometimes in post honeymoon stage partners subconsciously get in the habit of creating conflict/make-up cycles in the relationship to artificially create intensity when mutual compatibility is not there to produce said intensity on its own. This is pretty common in relationships where one or two partners have scarcity mentality and are afraid of just finding someone else instead of artificially extending the life of something that isn't meant to last on its own.
There's a lot of context that your post doesn't have, but I can't help noticing you seem to speak in terms of the things he gives you "He isn't as affectionate, isn't as sexual, isn't as loving, gets angry" or how his actions affect you "He likes to argue, and my feelings got hurt as a result of his actions" and there's some fatalistic talk that reflects fear of loss "he's falling out of love" "is the spark lost forever?". But your wording suggests you don't seem particularly mindful of his emotional state, as evidenced by the fact you didn't notice yourself rehashing old insecurities until he was already boiling. Maybe this is worth looking into.
You say you've both worked past your insecure phases, but it seems the main source of friction is bringing up insecurities. This begs the question: Are you truly past it? Six months is a very short time, far too short for any meaningful change on a personal level.
I've noticed that if a partner relies on the other too much for reassurance or emotional support on the formative stages of the relationship, they run the risk of having the emotional dependency dynamic either displace or corrupt the romantic/friendship one. i.e. you trade love for co-dependency. In a healthy dynamic there needs to be some degree of emotional self-mastery lest the psychological issues of the partners consistently upstage romance in the relationship. This is super common in online relationships also since text is the main communication medium.
Rumination isn't exactly an aphrodisiac either. How many people do you know that have sexual fantasies about broken records who endlessly crave reassurance about the same old bunch of insecurities?
You're not sure how to talk about these things, that's why you gotta ask him, not us. You say he accuses you of overcomplicating matters, and it certainly seems like having an internet forum triangulate this stuff for you is doing just that. Keep it simple and bring up this very uncertainty to him. This is how a relationship is built.
Also don't forget to listen with an open mind/heart. Not to dissuade, argue or excuse yourself, listen to understand him.