Success Plan objectives are CTAs. CTAs can be created, updated, closed via rules. But not Objective CTAs.
Please just add Objective to the CTA types you can interact with in the rules engine/JO or wherever rules are manipulated.
In addition to the numerous use cases you’d have for leveraging rules with CTAs, here’s one for objectives:
You have a process to handoff data from a sales team into Gainsight - based on that data you want to create a success plan, but the objectives of the success plan will vary slightly based on that data, as will the attached playbooks. Yes you can create multiple SP templates, but if you have more than one or two different combinations this will be obnoxious and not feasible to maintain.