I've used fish for about 5 years now, and I don't really wanna go back. Out of the box, without any addons at all, it does everything my old zsh setups did, with a package manager and plugins. The only change I've made to it was spacefish (now starship.rs)
You may also want to try z, and, err... I don't have my config with me, but I have a plugin that notifies me if a long-running background command finishes, which is really handy.
I use starship with zsh. The biggest value is showing the current environment for any project without any config. Shows the active node version, Python venv, k8s context, all without having to think about it.
The other day an AWS command was rejected with a generic 403 error and at a glance I realized I'd forgotten to switch the default profile—without starship I suspect I would have spent a while troubleshooting my SSO login instead of just switching profiles.
I also use starship with barely any custom config (like fish). It shows me the environment I'm in for a project, including which versions of tools are active and which server I'm currently connected to (if any). And it looks pretty enough.
Big one for me was out of the box support for showing current kubernetes context. Very helpful if you work in multiple kubernetes clusters and in particular if you move around namespaces helping others.
Doable with configuration and either lots of work or plugins in other shells but the combo of fish + starship.rs offers a lot for little setup.
While there is a tool called `babelfish` that will automatically convert bash to fish, in practice it's rarely needed.
You can run bash scripts from fish.
For sourcing, often it makes sense to use direnv to automatically load variables. -- For "just source this one time", in the worst case you can run a bash shell, source that, then run a fish shell.
