That means if I have several terabytes that I want to sync between devices, I'd need to have enough space in every device, is that right? (and each device would get a 1:1 copy of the folders). I read that when there are conflicts, the apps create a new "filename".sync-conflict file so you can manually resolve conflicts.

What I'd like is a secure way to backup all my files to some cloud provider (S3?) across all my devices, and have an MRU cache in a folder where I can configure the size (for instance, keep at most, say, 512mb of the files I most recently used). For things like photos, it would be better if the sync thing would get thumbnails from the cloud, so I would not need to download the whole thing just to find photos.

GDrive / GPhotos is the thing I use now. The combo has a lot of bells and whistles and I'd be fine having a simpler product where I could have more (perceived?) control on how things display, when syncing happens, etc (in lieu of other things like the nice A.I. photo search, integration with GSuite of apps, etc.). Kinda like using a file explorer on my local desktop.