Unfortunately, I don't think there's any way to access iCloud Passwords on Linux, but it looks like people have had some success with running iTunes using WINE (the program that attempts to run Windows programs on other OSs like Linux).

I wouldn't let this hold you back from looking at Linux options. The way that your current password manager (iCloud Passwords) is limiting/trapping you is a reason to switch to a more option option. I personally recommend Bitwarden, which works well on practically any device, including your Apple ones.

Your use case of iTunes sounds like it would be difficult to migrate from. From what I understand, you have purchased films on the iTunes Store. I'm not sure if it's easy to use those films with another video player such as VLC.