iMessage, iCloud, Auto-Delete, Keep Forever

Hi! I ended up disabling iCloud sync because I like keeping my message history for many of my conversations (some go back years). I was unable to figure out exactly how iCloud works, and I'm hoping somebody can clarify. I realize that when iCloud sync is enabled, any message I delete on any of my devices will delete on all of my devices. But how does this work with device level settings, like "Keep Forever" (on the computer) and "Auto-Delete" (on my iPhone)? At the moment, I have "Keep Forever" enabled on my computer and "Auto-Delete" enabled on my iPhone. If I enable iCloud sync, and have "Auto-Delete" enabled on my phone, will all of my devices lose message history that's over a year old, including my "keep forever" history on my desktop? And if I have "Keep Forever" enabled on my desk top, will all of my devices get that saved history?

(I wish iCloud-iMessage sync were a little easier to understand-- and that it had a "trash" so that we could restore conversations that have been accidentally deleted. 😕)