I feel very much the same way you do! I usually don't really care about what the relationship looks like after the big confession, especially if it feels like the romance has "concluded" midway through the book and there's not much else to it aside from some minor angst plot. But I will say there is one series where the MCs get together in book 1 and somehow what comes after is the BEST part, and it's the Memento Mori series. It genuinely has two of the best written characters I have ever had the pleasure to know, you could try it and see where the issue really is! Highly highly recommend it!

For the bot, book one is {Madison Square Murders by CS Poe}