For basic stuff (I'm the only one changing files, I'm not expecting to change a file on my laptop and have it on my desktop seconds later, I'm only mirroring files between Apple devices), iCloud has worked fine for me for a very long time.

For those reading this looking for alternatives: nextcloud-aio docker image on a NAS or small server works well for syncing files across machines. It's overkill, so I turn off most of the features, but I find the clients reliable across OSes (mobile and otherwise).

I do not put Nextcloud on the internet -- my use case does not require changes away from home to be live synced to others before returning. By the time I've walked in the door and put stuff away, changes + photos are synced.