It's definitely a work in progress, but AnyType has a lot of functionality similar to Notion. I haven't used it in a while, so I don't know whether there are plugins in any meaningful capacity.
From past experience, it's even pretty simple to host your own sync server to get away from their account/storage limits.
Fair enough, it's protocol is open source and the apps are source available. Modifications can be made by individuals for their own uses, though. I think it's as close as you can expect to get with a mostly full-fledged Notion competitor.
In any case, I don't particularly enjoy AnyType, despite coming back to it a few times to test it out (and still maintaining my own sync server, despite not actively using it, in case I go back to try it out again after some demonstrably updates). Just pointing out that it's a less restrictive alternative.
From past experience, it's even pretty simple to host your own sync server to get away from their account/storage limits.