I'd like to recommend Amethyst [1], it's a window manager that takes control of all windows, compared to Spectacle where you have to "move" each window automatically.

I've been using Amethyst for multiple years at this point, and it has been great for my workflow.

1: https://github.com/ianyh/Amethyst

At a glance, it seems like it could be a drop-in replacement for Spectacle, but with extra features.

Lets give it a try...

Spectacle doesn't even come close to i3.

The biggest deal breaker is that you can't programmatically move a window to a new workspace (I think?); and that you can't disable the animations for switching workspaces (biggest problem).

I would suggest checking out Yabai (https://github.com/koekeishiya/yabai) which is functionally similar to bspwm and allows quick switching workspaces (no animation or fade) and throwing windows to different workspaces or monitors.

Is it a case where perfect is the enemy of good enough? I've never felt the need to move anything programatically e.g automatically without it being triggered by my intention.

Spectacle has "next/previous display" shortcut.

To disable animations on workspace switching:

defaults write com.apple.dock workspaces-swoosh-animation-off -bool YES && killall Dock

That command doesn't work anymore since a few versions back of OSX. As far as I know there is no way to fully disable the animation and I hate it!

That command didn't work; and "reduce motion" from Accessibility still produces an annoying fade animation.

I recommend the chunkwm for tiling windows on macOS! It is super-fast and configurable.