The Infinite Kind advised in the past that the data stored in pre 2017 versions of the syncing files was only a subset of the transaction data that is contained in the main data file and is insufficient to rebuild the main data file.
Even though the full transaction data is in syncing files in Moneydance 2017 and later this should only ever be considered to be a last resort for the recovery of data i.e. after everything else has failed, you may or may not be able to recover from the syncing data, depending on what the original problem was.
I’m pretty sure the only file I have in Dropbox is the .moneydancesync file (but I’ll check when I get home).
If I set up the accounts from scratch on the pc will I be able to sync with my phone (via Dropbox) in order to update the transactions?
The Moneydance syncing file(s) is linked to a specific master file(s) by using a unique ID internally within the files, the unique ID is created when you setup syncing. It is not possible to associate an existing syncing file with a new main data file. As soon as you go to setup syncing on your mobile device the existing data will be destroyed.