u/DoingThings-

Is it balanced? What are some good eidolons and builds in general? Any other thoughts?

The beetle companion has the mount trait that a 35-foot speed. Slightly slower than the horse, but not by much.

The support action is the real benefit:

>Your beetle stomps around and smashes into foes, knocking them off balance or pushing them aside when you create an opening. Until the start of your next turn, while riding on your beetle, your Strikes that deal damage either make the target flat-footed until the end of your next turn or move the target 5 feet away from the beetle (this is forced movement). The target chooses which effect occurs.

Notably, this does not require it to be a Strike against a creature within the beetle's reach. Ranged attackers have a hard time getting off-guard, so this would be an easy way to get that. Also, knocking people away while ranged is much more useful than doing so in melee since you won't be knocking them out of your range.

Also, the advanced maneuver is just horse but better:

>The beetle moves at incredible speed. It Strides three times.

This is 105 feet compared to horse's 100 feet, so not that much better, but it can stack with other circumstance bonuses and also gets 50% more of a boost from speed increases compared to horse.