How to properly migrate user's email profile from local e-mail server to m365

My task was to migrate our HR manager's email account from our local e-mail server and keep all her folders, sub folders mails contacts etc. as they were. I have managed to redirect the mailflow to her new email address but when I was trying to import her old data (pst file exported from an ost file from her previous profile in Outlook 2019) something like "~not enough system resources are available to finish the process". I saw that her directoty structure had been already created, but the new ost file was significally smaller than the original. I gave it another shot and that was when things went south. I ticked the skip existing data button than proceeded with the importation again. It was completed with no errors whatsoever, but the new file is way bigger than the original now because it had duplicated every email with any attachement. Make things worse syncing with m365 was already turned on. Is there any way just to save the messages she had received since the redirection, but delete completely the new ost file, stop syncing and import her old profile back again, and add manually those few new mails and then turn syncing on in a way that Outlook on her notebook profile will overwrite her data in the cloud and not vica-versa?

You could create a new mail flow to save the messages she had received since the redirection:

Then you could follow the guidance to delete the .ost file: How to Rebuild or Create a New OST File in MS Outlook

Then you could export old emails into a pst file in old email address and import .pst file to the new email address. Step by step guidance for your reference:Import PST file into Office 365 – Step-by-Step

Try to remove duplicates, filtering those with or without attachments using this duplicate email remover or other utilities for finding and removing duplicates

If you are looking to move over all the history of each mailbox you could have a look at: https://imapsync.lamiral.info/

I used it a few years ago (not for local to O356) and yes it took some time to do each of the mailbox accounts I was working with but it did the job perfectly.