We’ve gone back and forth on the necessity, however with the majority of our clients transitioning the entirety of their work into Office 365 we highly recommend backing it up with a cloud to cloud solution. We have standardized on Dropsuite as a product offering for that. That is a channel only product, and multiple vendors in the CITN community offer it. We have tested many solutions from many vendors and found it to be the most complete, the most secure, and the most affordable.
If you are not interested in a cloud to cloud SaaS solution I would recommend looking into the built in free solution on Synology NAS or licensing the O365 backup capabilities from Veeam to do it in a cloud to on-prem solution.
I normally insist on not self-promoting on here AT ALL, however we recently have gained the ability to offer non-profit pricing for Dropsuite. If you’re interested in that product drop me a contact. To maintain fairness, other vendors that offer Dropsuite include Servant42 and Enable.
Dropsuite is licensed “per user” but once added to an org you are backing up all Sharepoint, Teams, Shared Mailboxes, etc. Generally, we want to license it for all actual staff/active users so we are protecting all email and not pushing the limits of reasonable use on the storage.