So here is the thing I struggle with. I have a NAS. On it is my media (music, videos, photos) collection. I do not have a subscription to Apple Music, or the reset of the streaming services and I not want one. So how do I play my own music collection with this?

Right now to do this I have an AppleTV on every TV. I have an app on the AppleTV called MrMC which is basically a paid for port of Kodi for the AppleTV. The MrMC app can mount the NFS file share and play my music just fine. The AppleTV is plugged into and HDMI splitter that strips audio and outputs it to an amp which goes to my speakers. This just works, and I can remote control it with my an app on my iPhone.

Given that the HomePod will not be able to access my music collection, it is useless.

I would be happy with the HomePod being able to steam music from an open Music app on a Mac somewhere. I have enough spare Macs that it would be simple to just run Music on it (which mounts the network share.) But of course, I would be wasting power.

Oh well.

It would also be really nice if the AppleTV would just support SMB again.

NAS -> Plex -> Nest Audio (or other Google speakers) would achieve this. If your NAS doesn't support Plex, you could easily run it on a Raspberry Pi if it's just for music.

You'd have to look at alternatives for the media server (Plex) if you want to use a Homepod though .. I'm pretty sure Plex doesn't support AirPlay.

If it's just wireless audio you're after and not a "smart speaker" then there's some not-too-tricky ways to do this in the DIY space. snapcast for instance supports AirPlay and might integrate nicely into your current setup: https://github.com/badaix/snapcast