Astrologian is actually one of the more enjoyable classes to say, and overall I don't think there's much I would change (within the confines of reason).

Making Combust the same as Aero III wouldn't really change much of anything for Astrologians in my opinion, besides maybe having to press it a bit less. I don't think this would provide much in the way of quality of life.

Unfortunately, removing the duration of Synastry to make it essentially infinite would make Astrologian far too powerful.

I'll admit I don't often use The Spire outside of expert dungeon AoE pulls, but I don't believe a movement speed increase would make it much more appealing.