I don't need so much storage on Dropbox. I get by with the free account, plus some extra that they've given me over the years. My main storage is on Google Drive. I don't see any Dropbox plans for $17.50/month. You can get 2TB for $10/month, and you can pay annually.

I have set up syncthing on my desktop, laptop, Pixelbook and phone, and I'm syncing only one folder of essential files for now. I'll see how that goes. I'm mostly at home these days, and I get by with NFS. All I really need synced is my keepassxc database. I've been doing that on Dropbox for years, but if I can sync reliably on syncthing using my own devices I may go that route. My real issue is the inability to mount the NFS drives on my phone and Pixelbook. I don't have a lot of available storage on them, so it would be nice to be able to mount the NFS drives on them. It's easy enough to mount Dropbox and Google Drive, but not NFS. There are also clients for samba available, but I don't want to use that.

I would not be surprised if that's a 'hook' price, and once you've started using the service and they consider you 'locked-in' that # goes higher and higher - just like a storage unit, or many other 'as service' products you can buy these days...

As far as the phone mounts - I'm not really very familiar with Android but there doesn't exist some 'app' to enable basic linux functionality on the otherwise linux-based OS? I know on macOS, for example, SFTP is available as a FUSE plug-in.

The Dropbox site has a lists of all plans, with pricing and details. It has been in place for a long time. It's not a come-on AFAICT, that's the regular price for a personal, one user account. There are family and business accounts allowing multiple users for more money if needed.
https://www.dropbox.com/plans?_camp=...ink&trigger=nr

There are Android apps for SFTP, FTP, and Samba. Nothing for NFS. I have searched.