Thank you for helping out. I have browsed through this 2-way sync but strangely, for me it does already work 2-way for the “photos” folder. But not for the “Backup” folder. I tried a few different things (like different ways of creating that folder) but still no auto-upload from the phone.
As a workaround, rather than installing that paying solution, I’ve decided to go for manual upload… 
good enough in my case.
But definitely frustrating (if only the /e/drive sync allowed more fine-tuning for files, none of this would be needed).