There's a lot of other proof assistants out there, like Mizar, Isabelle, Metamath, Metamath0, and Coq, each based on different foundations. Metamath0 in particular looks really intriguing, since it's the brainchild of Mario Carneiro, who was also the person who started Mathlib (and I believe is still an active contributor).
Anyways, Lean definitely has a lot going for it, but I love to tinker, and it's interesting to see other proof assistants' takes on things.
Most people seem to be rallying around Lean these days, which is powerful and quite featureful, but with tactics metaprogramming feels more like writing C++ templates instead of the "assembly language of proof" which I liken metamath to, for its "down to the metal" atomization of proof into very explicit steps. Different (levels of) abstractions for different folks.
Once I return to a proper desktop I will probably woodshed myself into Lean for a week or two to get a better handle on it, but for now tactics feel like utter magic when not just chaining `calc` everywhere.
You might find metamath0 interesting, its kernel design has a similar focus on simplicity while cleaning up a lot of metamath's cruft: https://github.com/digama0/mm0
EDIT: and feel free to ask any questions about mm0, I don't know a ton about it yet but I have researched it a good deal. I'm hoping to use it more this fall when I take a class on first order logic and set theory!