This works, but only if the partitions are next to each other on the disk map. I merged a 200GB APFS partition with a 100GB empty HFS+ partition behind it — seamless, took 30 seconds. Non-adjacent partitions cannot be merged (a common technical limitation).
By the time his coffee was cold, his Mac was reorganized and breathing again. The beachball was gone, replaced by a streamlined workspace ready for the next edit. stellar partition manager for mac work