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.