Okay, you are probably already aware but it’s important to remember that there are two OneDrives: the personal and the Sharepoint shares (group shares). In Windows 10 as of 1709 you don’t necessarily sync the Sharepoint files so you won’t have a local copy of those to backup unless you expressly set it to sync to a computer someplace and expressly back them up. If you don’t deal with Sharepoint then that won’t be an issue, but if you do any group shares then you’ll have to factor that in.
I’m a Cloudberry partner and have it deployed en mass but I have an ambivalent relationship with Cloudberry and may not renew (some of those reasons are very MSP specific so that shouldn’t deter you). There are several ways that Cloudberry can operate and it will send you emails when it fails… and sometimes when it thinks it fails… or if Google Drive is in a user profile and system can’t access the files to backup… it can be a tad annoying in that regard when managing at scale, but I digress. Anyhow, you can do differential backups, at one point I thought I should do them every time a file is changed, but that was a bit too much overhead and now I just do a daily differential backup each day with a 30-day retention and it’s been satisfactory. You’ll need to occasionally run a scheduled full backup though or those differentials can grow too big, at least once a quarter in my experience. That being said, you can alternatively just use the cloudberry backup for Office 365 and dump the backups into Backblaze B2, it’s not local, but if both MS and Backblaze completely die at the same time then our church office probably isn’t safe from that disaster either.
You should also take a look at Veeam, they have both O365 backup and they have a free backup agent for Windows/Linux if you are backing up to a network drive. I always use my RMM to monitor for backup failure via the system logs so I’m not sure if Veeam to network share will send an alert on failure, but I’d recommend an RMM approach in general to all your alerts.