Sand-boxing Issues

New Account as I have some players (that are also DMs) that know my main.

I started running this original setting a year or two ago, it was intended as a sandbox campaign. I had custom tables and all that but my players also expressed a desire to be able to really get to know the various NPCs in the places they went to. So, I would plan 10 or so npcs at a time all with connections to things an and out of the town with room to create more folks as needed, I would draw a map of the place, find or make portraits (players like to have a picture of the npcs)- all the nitty gritty stuff to make the town feel Alive and ripe for relationship building and whatever missions the players wanted to take before eventually moving on. I would do this prep between sessions so when they would say they were going to X place, I would plan that place and maybe the next town over as well.

They would roll into town, learn the names of a few locations, buy some gear, and then leave. This happened consistently. I tried to have more random events spring up for them but they would be avoided more often than not, and NPCs continued to be ignored or suspicion would be poured onto the friendlier ones before the players skipped on out of town. I started ending sessions early because I didn't have more stuff planned for them to do, having assumed they'd be in an area for at least 1 or 2 sessions at minimum before moving on, seeing as that's what they made it sound like they wanted! They expressed frustration at my ending sessions early, and I told them that I had all this stuff prepped that they chose to avoid. Despite explaining to them that's where the content was going to be, and I would do it based on their journey, they still avoided it.

I ended up switching to a more traditional plot and just zoomed them through the sandbox on a literal train track to the adventure, plopped them in one big city, and made them stay there/brought the adventure to them. It became one of their favorite adventures and they were always asking to go back to it.

This is important to bring all this up because we started a new game in the same setting, and now their new characters are leaving the city where the adventure is, and wanting to do more 'getting to know the locals and road trip' stuff. I would recycle my old locations- except they want to go to a whole new continent.

How the hell do I do this without going insane? They expressed interest in setting down eventually in a town, but how the fuck do I plan enough Sandbox Stuff to let them float from town to town, while also making sure they town they settle in will be interesting enough to get them to actually stay?