Set-SmbShare is hard to use to modify permissions on a share. It is also inconsistent with how New-SmbShare and Get-SmbShareAccess handle permissions. There is no straightforward way to get the current permissions from a share, modify them, and put them back.
Get-SmbShareAccess returns a table/list representing the permissions, but Set-SmbShare -SecurityDescriptor requires an SDDL string. Translating between this is too much to ask to automate an IT Pro task that can be done easily from the GUI.
This should be improved. For example there could be a Set-SmbShareAccess which works with a data structure compatible with what Get-SMBShareAccess returns. Also, Set-SMBSHare could gain easy to use permission granting parameters like New-SmbShare has.