You can not have overlapping partitions that are both attached. Why do you want to merge partitions that you are "done with" instead of just leaving them partitioned by day?
Why are you partitioning at all? Are you confident that you need partitions for performance & that the trade-offs are worth the cost, or are you needing to detach/drop old data quickly to adhere to a retention policy? pg_partman extension may be worth looking into. I have only dabbled with it, so I can't offer much comment.
