And if some tool you use doesn't have completions, writing completions for fish are infinitely easier than bash

It's not difficult to customize. IMO, it's actually easier to customize yourself than bash/zsh because I find the fish syntax much more natural than bash, but YMMV.

That said, I believe that zsh has a bigger community, and therefore most likely a richer ecosystem with more plugins. For example, oh-my-zsh has 142k stars, and oh-my-fish has 8k stars.

However, even with the rich ecosystem of plugins, I wasn't able to get my zsh to be as pleasant to use as my fish, where I only have some self-made customizations (not even using oh-my-fish).

But if having lots of plugins to choose from is important to you, zsh may be a better choice.

I'm a previous bash user who now uses fish and loves to customize everything, and compared to bash, customizing fish is a joy. Its so much easier to do, using oh-my-fish brings a great selection of plugins (admittedly lots of them are inspired or clones of oh-my-zsh plugins) and writing plugins is a breeze as well.

Very easy especially with omf, has a web interface for terminal schemes. I think you’ll be right at home.