The variety of experiences here illustrates the fact that iCloud is a massively distributed system. Between the user and cloud storage are many intermediate services, protocols, and equipment. E.g. Wi-Fi, cell service, ISPs, etc. all of which can glitch, or have bugs, or engineers tinkering with things.
I don’t know the answer to this uncertainty, other than try different services and keep backups.
I use my own Nextcloud server for sync, in addition to iCloud for iOS apps.
I checked on pricing (a little), and from what I can tell, Dropbox is by far the cheapest at $10USD/mo for 2TiB. I don’t like that Dropbox mandates a name for their folder. As far as it being hooked into macOS, I think that’s going to be true for most of the commercial apps (including iCloud), as that’s how they make syncing transparent.
Box 10GiB free, $10/mo for 100GiB
Nextcloud on webo.cloud 5GiB free, scaling up to €31/mo for 1TiB. Looks like all Nextcloud providers are outside the US. As I said, you could also host your own at home, or use a provider like Linode.com to set up a server.
You can also buy a precondition box to run at home Devices - Nextcloud, use a Raspberry Pi, etc.
If you just need syncing and not cloud storage, syncthing.net works well for me, and is free. You could conceivably set up a cloud server of your own running syncthing for cloud storage.
I would recommend having a Chronosync, Carbon Copy Cloner, et al. task that copies iCloud folders to some other folder (perhaps an attached external drive), so that e.g. Backblaze will back it up. (Which I’m about to do because I just thought about it ).