How to: large photo archive on Windows, want to synch with Google Photos but store in a specific Photos folder.

I've been using ST for ~6 months and sync camera folder on my new phone to an old phone with UL photo uploads, works great. What I need is to import ~10k photos from external drive from a relative, but would like to have them synch in google photos to a specific folder so my relative's photos aren't interspersed with my own photos since imports will read the photo date and sort them accordingly most times in google photos.

I'm guessing I need to synch a desktop folder on my computer to my "new" phone's camera folder that is synched to my old phone and that gets me uploaded and free storage, but the photos would be all over the place. Thanks for any ideas!