Hi Marc,
yes backup is an issue with "Optimised Storage" because you don't have full quality photos on your internal hard drive.
The only way that I know is tricky and requires attention.
Create a new user account on your Mac, "Backup User"
This user will have the same Apple ID as you use normally.
For this user the system photos library would be on an external drive, you would load the originals to this library.
Please read this article to setup the library on an external drive
Move your Photos library to save space on your Mac – Apple Support (UK)
Follow exactly the definition for the format of this drive.
As long as this "Backup User" will be logged in every few weeks you have always a second copy of your library with full size photos.
If you like you could include this external drive in your Time Machine backup, I would recommend that.
As I said, this is not very elegant but I should do the job.
Pick the size and speed of this external drive based on your amount of photos / videos.
I would use an SSD, just for speed. But a regular HD would also work, because you are not frequently working with the data on this drive.
Ralf