Rectangle (https://rectangleapp.com/) is Spectacle's successor, works great, and its source is available as well (https://github.com/rxhanson/Rectangle).

+1 for BTT, particularly in combination with Karabiner Elements and goku to handle Karabiner configs in a good way.

If you like tiling window managers, you can get a very similar experience (in terms of using the whole screen and being able to flexibly throw things all over the place) with this kind of setup.