Hi @DWhitlow,
I feel your pain, I really do. The move to the new GUI on ONTAP has been a slow and arduous journey (started in ONTAP 9.7 from memory) and the new System Manager GUI has no-where the feature set of the classic GUI.
Unfortunately even in ONTAP 9.16.1 you cannot using the GUI to create a LUN and place it on a volume that you want other than the "Group with related LUNs" feature where "The LUNs will be grouped together with related LUNs on an existing volume on the storage VM." This means you need to know an existing LUN name when creating a new LUN. Details here https://docs.netapp.com/us-en/ontap/task_admin_add_luns.html
Creating a volume per LUN is generic "best practice", but then you need to be aware of scaling figures and the like (e.g. maximum volumes per array/cluster)..... so I understand why it is the default.
To be honest the best way to move forward is the CLI, sadly, and you should just save your commands and build it out into a template when you need to add new volumes and/or LUNs.
Why did NetApp simplify its GUI? In my opinion it would be due to the pressure of competitors that have simple GUIs that abstract away the underlying storage so general system administrators are able to maintain a NetApp vs dedicated storage engineers. NetApp was losing market share as they were too complex, too many "nerd knobs" when people just want it to be simple, fast, and keep out of the way and provide storage to their applications........ Well my opinion anyway.