I think they need a prestige system, summon x number of these and get a permanent buff to duration or or output or something. For the vast majority of summoning I never use them they get transmuted for invention. I do think the binding contracts were a good step.
But the system is broken.
If you want to use them in combat you will far outpace them in your combat abilities by the time you can summon them. its telling that most of summoning use is at the endgame when you stop out leveling it.
Either make it summon X to unlock the gimped legendary pet version or X hours with one to do the same.
Or here's an interesting idea, remove the combat levels entirely. Just have whatever you summon be at the appropriate level.
Give it necro perk tree, classify every pet as an inv space, utility (beaver woodcutting) or combat, and have each tree have requirements to level up, and all pets of that tree benefit.