I think I see what you mean.
so what I could do is pair to HomePods in stereo from Apple TV and then use optical/hdmi out to feed AV receiver and use that to power Center/Sub/rear etc, can’t see why that wouldn’t work
You'll run in to at least two problems. These are:
(1) controlling the volume levels of two devices because both Optical and HDMI have fixed output levels, so you'll end up trying to match the AV receiver volume and the Homepods volume every time you make a volume change
(2) latency. ATV4 to Homepods and any other AP2-capable speakers will all be in sync, but that doesn't mean that the Optical out or HDMI out will share the same sync. It's not designed as part of the AP2 ecosystem, so there's no reason for it to be in sync.
Added to that, the AV receiver is already coping with picture-to-sound sync to do with the time the TV takes for its picture processing. It's only my guess, but you may end up with the centre/rear/sub audio being in sync with the ATV4 picture, but the stereo pair of homepods being either in-front or behind and no way to bring that back in since the AV receiver's audio-video sync control is already busy compensating for the TV processing time.
Next, you'll probably also have an issue with duplication of the front channel (L,C,R) audio.
You see, the front stereo pair will carry the centre vocal along with stereo music plus effects; so now you have voice coming out of the left/right speakers as well as the centre channel via the AV receiver. It may also be out of sync. As jpaul pointed out, the stereo mix of a 5.1 audio source will carry everything except for the LFE channel's deep bass. I suspect that there's no way to tell the ATV to send just the 5.1 version of FL and FR to the homepods.
It's all very messy.
Do you already have the Homepods; is that why you're trying to make this work?