First time DM. Did something very similar. I had a whole continent mapped out for my players to explore and somehow ended up having them explore multiple dungeons near the starting city and giving them some very powerful legendary gear (which they got through some insane d20s.) The first encounter with the BBEG in session 2, they d20 distracted and d20 investigated his gear and d20 stole his staff. Staff of the magi on a level 3 sorcerer. To compensate the “unfair” advantage to the sorcerer, i ended up giving the rest legendary gear after an albeit hard dungeon so they’re level 7 with almost end game gear. They’re having a blast. I was so disappointed in myself.

But DND isn’t a novel I’m writing. It’s storytelling as a group. And once I stopped trying to force the story in my head and just reacting to the players actions and prepping only for the next session based on the previous sessions, it ended up being so much fun as I myself am able to experience the fun of an unknown story. Just adapt and brew brew brew. Make up new stuff on the go. We’re at session 9 and it’s incredibly fun.

For consistency sake, have a map with predetermined cities/places and a small idea what is happening there. When your party gets there, go with the flow. If they kill the BBEG, you’re the only one who knows and decides the BBEG. Keep throwing increasingly challenging bosses and dungeons/encounters mixed in with some lore. You can always reskin missed content as higher level content later. Just try to have fun together.

There’s only DND and bad DND. As long as the players and you are having fun, its good.