Technically, it is no problem to expand a SFS partition within the generic maximum partition size restrictions, just the related number of bitmap blocks needs to be expanded and the 2nd rootblock on the very end of the partition needs to be rewritten.
We might provide a tool for that in the future, for now you can use SFSDoctor to sort of fix it as a workaround.

MorphOS 3.20 will also introduce a new SFS DosType to get rid of the current 128GB-1 partition and 4GB-1 filesize limitation, so some way to convert old SFS type partitions to the new format might be reasonable in the future as well.
For now, 3.20 will only provide adjustments to the known SFSDoctor tool to deal with checking/scanning partitions using the new SFS DosType and salvage/repair files if required.