Apple - Can my employer access my iCloud data if I sync it on my managed Mac
With a managed Mac where "everything is controlled by the employer", the employer could technically access your personal data stored on that Mac - yes. It doesn't matter if the data comes from iCloud or other places.
Note that your employer probably does not access your personal data, and depending on where you live and what you agreements are, it is probably not legal for them to do so. However, it is very probable that they could access the data given the necessary intent and technical knowledge.
My advice would be not to store any personal data on a company laptop, unless you wouldn't mind your employer or a system administration to stumble upon that data.
From your personal iCloud account share a folder with only work-related files to a company supplied AppleID/iCloud account running on the managed MacBook.
At a moments notice your work-owned Managed MacBook could be deleted or locked.
It is their property and possible agreements regarding its use may include any data on the machine.
Why else do you need your personal information on the work Machine? Use your personal machine for personal stuff.
You will probably be a much more productive employee when you are not accessing your personal stuff when at work, or when working.
Of course they can access. They can potentially access even if you delete your synced data, because your data might end up being stored in system logs, cache, hidden folders, and even deleted data can be recovered with special software.
I myself recently stumbled upon my old work emails from two years ago on my personal Mac. This is despite my work login has been disabled for two years. But I haven’t used Outlook on my personal Mac since then, I recently opened it and it restored all my work emails from some archived folder.
Don’t share your personal folders either. Create a shareable folder from your work account and share it with your personal account if you need to access work files from home.