I’m wondering if there are possibly any tools out there to do this: Declare a color scheme and set them to all apps CLIs and TUIs, either immediately on upon restart of each app individually. I looked at Stylix but It seems like I have to rebuild the configuration every single time I want to change the color scheme.
Yes, that’s how nixos config works.
If you don’t want to use nixos config for this…
Uniform look for Qt and GTK applications - ArchWiki only covers GUIs, and TUIs are never going to be consistent with each other, much less have the same colors as GUI apps anyway, so 
depends on what you’re willing to tolerate. You’ll probably have to theme TUIs individually.
You could have a look at matugen and this repo that uses it quite extensively: GitHub - mylinuxforwork/dotfiles: The ML4W Dotfiles for Hyprland - An advanced and full-featured configuration for the dynamic tiling window manager Hyprland. Ready to install with the Dotfiles Installer app with setup scripts for Arch, Fedora and openSuse.