Doesn't your iPhone capacity get full up? Can you selectively choose what is synced? Last time I tried (a few years ago) it was all or nothing
You can set this in the photos setting. You have the option to store all content locally while also backing up everything in th cloud. However, none of my devices have the space for 357GB of photos so I default everything to “optimize local storage”.
"Favourites" seems to -almost- have this priority in that they'll more often than not be on the device regardless of age and whether you've specifically downloaded them but it's not -quite- there.
You’re gonna be fine :)