From dropbox to b2, modtimes, --checksum and update-modtime-only?

I’m migrating my backups from Dropbox to B2 and crypting at the same time. So far so good but ran into a few small issues.

I guess this has to do with the fact that Dropbox doesn’t support modtime I think?

When I do rclone copy dropbox:myfolder b2crypt:myfolder all is good

Then I do a rclone sync local:myfolder b2crypt:myfolder and I noticed that it recopied a bunch of files which hadn’t changed. So I guessed it was something to do with the modtime.

So I repeated the whole process this time adding the -c (–checksum) flag and sure enough it didn’t do it this time, the local sync only sync’d the new files that have actually changed

However I would like to not have to use the -c if possible after this initial transfer.

So was wondering if there’s an option to basically “sync” the modtimes only. I don’t want to have to retransfer an entire file if the file hasn’t changed but only the modtimes are out of sync.

Is there a way to do this?