I think you can select all folders, I recommend taking the path I did. Run Immich in parallel with Google-Photos for a few months and see if you like it.

I did so an synced my camera roll from my phone (~6000 photos) and it works great. I also added some family members etc. and they also like it.

Now in the process of moving everything over.

It would be great if there was kind of cleanup plugin for Immich.

Like Tinder for pics you want to keep or dump. I could spend 10min every day in the train to swipe left or right on pics I want to keep or remove.