3. Is my assumption correct that if I run SiYuan on Docker and have S3 or WebDav sync enabled, I can sync with my iOS client? I somewhere read that the docker version can't be synced with iOS, I can't find the source of that information again, but this was probably only meant for "direct sync", S3 should work, right?

You can sync between iOS, Desktop, Android, and Web (docker) instances if you have purchased at least the Pro lifetime license via S3 and WebDav.

I'm syncing with a self hosted instance using Minio for my S3 compatible provider and so far it's working very well for syncing between my laptop, desktop, and mobile devices. My workspace is not nearly as big as what you are considering though.

I noticed the initial sync after setting things up for the first time took several minutes even though there was very little to sync, so you may want to set up sync before you add tons of content to the workspace.

In addition to that the column must have the "Wrap Column" setting enabled or it will not honor the line break in the actual display of the column.