Shouldn't this process be automatic based on some heuristics?
Saving 10ms planning but costing 14s execution is catastrophic.
For example, using some statistics to limit planner time to some percent of of previous executions.
This way, if query is fast, planning is fast, but if query is slow, more planning can save huge execution time.
This is a better general usage option and should be enabled by default, and users who want fast planning should set the variable to use the generic plan.