There are zero audio sync issues between speakers all wired to the same receiver. You can get an input cable that connects to a headphone jack to make an iPhone resemble a CD player input on the back of the receiver. Also need an apple dongle for phones w/o headphone jacks.

The very mild inconvenience of using different speakers in different rooms to me is very tolerable vs. the horrible experience with my Sonos speaker deciding to stop working or can't connect, forcing updates, etc. when I just want some chill vibe background music.

Have you tried recently? I’ve been playing with music-assistant, snapcast, and shairport-sync on some raspberry pi zeros. Synchronization has seemed to work incredibly well during my experiments.