One thing I sorely miss after switching from Linux to Mac is i3. Spetactle gets close, but something as trivial(?) as switching window focus with the keyboard I couldn't figure out a solution for... Can Keyboard Maestro do it?

There are a couple of automatic tiling window managers for MacOS, the most notable being Yabai (https://github.com/koekeishiya/yabai) and Amethyst (https://github.com/ianyh/Amethyst).

Yabai doesn't handle window switching on its own but it can use SKHD (https://github.com/koekeishiya/skhd) or any other application that can bind terminal commands to keyboard shortcuts such as Hammerspoon (https://www.hammerspoon.org) or BetterTouchTool (https://folivora.ai) or even Keyboard Maestro. The commands are context-aware of spaces and the placement of windows on the x-y plane so you can move between windows relative to their position on the screen.