The audio sync is really hard to get right on homemade setups (I tried).

Sonos really sucks at software, but they get that right. And the speakers are good quality.

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.

Sonos syncs things up well-enough that a pair of them can be used in stereo. The stereo pair will play in-phase (allowing for imaging like with conventional stereo rigs), and stay locked together even as devices are added and removed to/from a group.

Do DIY solutions accomplish this?

For me it was more about the cables than the smart features.