Counter column in document set

Hi all,

i've developed a document set with a column "number"  incremental by flow; it work but issue is that when i insert new items, i've to wait the flow to start before i could get column valued and all the files created in document set have column number blank.

To solve issue, i need column "number" valued as soon as new document set is created.

I can't use powerapp because doesn't support creating document set.

Any ideas? spfx?

Thanks