I think it's fine to exclude the Exchange database(s) from the image backup. You'll have to restore the Exchange DB after a full OS restore, but as you stated, you'll reduce the image backup time and storage needed.

Thanks for you quick answer!

Do you think it's better to exclude only the EDB file, or the whole "x:\Exchange Server\v14\Mailbox\Mailbox Database xxxxxxxxxxxx" folder?

I am not an Exchange expert, but my understanding is there are three main file types in Exchange in the Mailbox Store and the Public Folder Store: EDB database files for messages, each with a corresponding STM file for email attachments, and LOG files which contain all the activity that is taking place. I think those can be eliminated.

Having said that, I do not know what the full server restore process would be like from a complexity perspective as Exchange may need to be configured in some way to get it ready for restore after the OS image restore completed (as it's missing the Exchange database files). I would highly recommend if you go this route that you test your OS + Exchange restore process on a virtual machine and document the process carefully to ensure you don't run into restore issues with your company (or your customer) should a full server restore be needed.