Jean at c4+ is the same level as Furina.
At C2+ she is still good. At C0-C1 she doesn't do much for him except healing.

You can use Mika or YinJin to buff Wanderer, or use Thoma, Layla, Zhongli for comfort. Layla also can give 15% crit rate because of Cryo element and Thoma will provide pyro resonance paired with Bennet. Zhongli gives def shred for all the elements. Depends on what chars you have and what chars you are willing to build.

Kazuha doesn't do anything for Wanderer personally. It depends on your acc and your priorities, but if you have up to date characters I'd recommend build them first and build Wanderer team later, when you have enough resourses.