obsidian's cool but it's not keep-like at all. don't know what would be a good alternative off the top of my head but i bet there are tons.
I recommend Joplin. Doesn't have Obsidian's fancy UI or add-ons, but the core functionality is the same. Should be great for the vast majority of users.
your best bet is fdroid obviously but in terms of apps available it's lackluster af and the UI looks like android gingerbread or something.
They recently overhauled it and it looks mostly modern. Still not perfect, but definitely a lot better than it used to be. There are also third-party front ends. Can't really say anything about them as I haven't used any.