"Getting Back In" vs FOMO

Hello! I've been a fan since the first game and played this one for a while, up until a few seasons into Warlords of New York. I'm thinking about getting back into it, but I have a question!

This might sound crazy, but I actually really like the lore and narrative of TD2. I'm worried that coming back in, I'll have missed out on a lot of the ongoing story that's continued through the seasonal content.

At this point, is it possible to go back and catch up on the story/narrative missions from previous seasons, or is that sort of a "FOMO" element that's no longer accessible for new or returning players?

Thank you, and **Extremis Malis, Extrema Remedia**!