SpacetimeDB is open source[1], the open source version doesn't currently have all the features of maincloud, but most of those features are solely for scalability. I've done all of my work on my own local servers. I would even suggest paying for your own server up front until you're ready to scale out. I believe the 4 years is when the license changes to a more open license. And the scalability features on local are coming, they're just not here yet.

[1] https://github.com/ClockworkLabs/SpacetimeDB