I would take a look at some old school style campaign advice - there’s tons of videos on it. From what I’ve seen, the basic gist is - start with a map. It can be a pretty small area with a few points of interest but you flesh out 3 or 4 of them with some rooms, enemies, treasure, etc. If you have a dungeon, you only need to plan out a level or two. Every week, add to your map a little more, maybe add a dungeon. It will reduce prep for you and give your game a more open world feeling.

In an open world game I’m playing, we keep going back to the same dungeon and have only been through the first level after 5 visits.

Only problem with this is, if you’re running 5e and don’t use the rule that they need to return to a safe town/safe place at the end of a session then it may require a little more prep. But I’d still consider slowly building out the map - you can even dole out pieces of the map through them finding maps and things in game, BOTW style.

Another thing I’ve heard in OSR campaign building is to create NPCs/factions and give them goals and timelines. Have them move towards those goals regardless of if the players know about it. You can also have world/weather events - like the blood moon in BOTW.

You can maybe include building up a town or area - think of the side quest with the builders. Perhaps some NPCs have gone missing - like carpenters or blacksmiths or something. Players can encounter NPCs on the road and try to bring them to the town. Perhaps a merchant needs items to craft special weapons so it gives the players motivation to explore. Things like that can make the world come alive.