Thank you for your comments. They are very useful.
Since WinSCP can encrypt the files and also do synchronizations using Windows Task Scheduler, it should most likely satisfy my needs.

Given this starting point, including the fact about Google Drive not being supported by WinSCP, I'll go away and do a lot of reading. I'll try not to bother the forum unnecessarily with other questions unless it's absolutely necessary.

This is all quite broad. Just few notes to your individual points.

1. WinSCP supports on-the-fly encryption of transferred files, but with SFTP protocol only.
   https://winscp.net/eng/docs/file_encryption
   Or did you aim to encrypt the file yourself before the backup? I'm not sure how you mean it.
   
2. WinSCP supports synchronizing.
   https://winscp.net/eng/docs/task_synchronize_full
   
3. You can schedule WinSCP synchronization script using Windows Task Scheduler.
   https://winscp.net/eng/docs/guide_schedule
   
4. WinSCP can preserve deleted files and overwritten versions of files. For for overwritten files it works with SFTP only.
   https://winscp.net/eng/docs/ui_login_recycle_bin
   

Note that WinSCP does not support Google drive.
And OneDrive is supported via WebDAV API only, which has file size limitations.
https://winscp.net/eng/docs/guide_microsoft_onedrive

In the course of looking for alternative ways to keep a data backup off my premises, I came across WinSCP and I’m considering using it for this purpose.

The facts of my case are as follows:
Windows 11 21H2
Laptop with 8GB RAM
Data to be backed up: About 24GB on C drive and almost 150GB on a USB-connected external disk.

My aim is ideally to:

1. keep encrypted backups on the destination using my own encryption key
   
2. synchronize the source and destination files
   
3. schedule when the backups would take place and
   
4. keep versions of my backups, that is versions of files that are deleted or replaced on the destination during the transfer from the source.
   

I’ll probably want to use either Google Drive or OneDrive, but I’m open to other suggestions provided they are not too expensive.

I fear that what I’m looking for is probably too much and maybe a few of the above objectives cannot co-exist. However, it would be useful to know what is achievable using any combination of the above.

Any suggestions would be most appreciated.