I think for preexisting solutions, the "best" one is Rectangle Pro, but it isn't free, so maybe that doesn't count. That said, eventually I realized I don't even want the whole "window split" stuff and I'd prefer to just have a few keybinds that throw windows into specific coords on my screens, so I installed Hammerspoon (free) and wrote a screen's worth of Lua to do this for myself. It is written for my two adjacent 1440p monitors and personal preferences, but the code is really obvious so if you're comfortable with making your own bespoke solution, this is pretty nice, and free.

* https://www.hammerspoon.org/

* https://gist.github.com/joedrago/bfc54f4083b070fe998d519cc6c...