I wish I knew as well. I’ve been using Chromecast Audio myself, which works with PlexAmp self-hosting my music.
The problem is Chromecast Audio has been discontinued for years of course - Google did their Google thing, and unfortunately I never found anything else like it on the market. But you can connect those devices to any speakers and sync multi-room high quality audio very easily. I managed to pick up 4 of them when they did their fire sale, and I think you can find them on eBay for now still.