I do pretty much the same thing, though i backup locally to a NAS via TM, and remote to OneDrive using Arq.
Like you, i've also never had to restore, but i somewhat frequently (every 30 days) verify my backups, and i've never encountered a problem with Arq. TimeMachine also appears to have become a whole lot better at not corrupting your backups every n days, but i guess that's APFS at work for you.
The one remaining thing i have left is to figure out the "proper" way to backup iCloud only storage. Currently i have an app on each users device that uploads photos to a NAS, and each users laptop then backs up that share on the NAS to the cloud when Arq runs (i use scheduled snapshots on the NAS so no backup NAS to NAS for local).
While it "works", i don't completely trust it, and i would much prefer a better way. I'm aware i can put the photo library on an external drive and download originals, but that's not very practical for people that bring their laptops with them, and also access photo libraries on the go.
The only other solution i've come up with, is to have a dedicated "always on" machine, where each user logs in through remote desktop, and have that download originals for all users, but it requires me to remember to ask each person to login to the machine every time it's rebooted, which again is a chore, and i much prefer something automated that runs on the persons own devices instead of a centralized setup.
I actually broke down and I'm paying the exorbitant Apple tax for the extra iCloud storage for photos etc to deal with syncing between phone/mac/ipad etc.
Oh i'm paying that as well, i'm talking about backing up the photos stored in iCloud, and preferably in an automated way.
Apple's official documentation[1] simply says to login manually to icloud.com, select the photos you want to backup, and download a ZIP file. I have 3.5TB photos in iCloud...
It's enough that i can't even purchase a Mac that has the needed amount of storage, nor should it be necessary to mirror all data locally before backing it up. All i want is a way to backup my iCloud Photos locally without storing them twice (locally). It works for Documents in iCloud, and Arq handles those well, but because Photos is "special", when it stores "optimized versions" of your photos, it actually replaces your local versions with lower resolution images, so even if you backup your iCloud photo library, you still don't have all the data.
<fingers crossed> haven't had to restore, but have verified the backups.
The problem I had with Time Machine is that I wanted to configure what to backup and avoid some things like work vs personal etc.