Thread and HomePod: HomePod Does Not Support Thread, Takes Down Thread Mesh - Take 2

Posting as the answer:

A solution that worked here is to put a Thread-capable HomePod Mini (aka MiniPod) on a seperate local network in the house. I have an eero based network (with an eero 6 gateway acting as a Thread Border Router). It supports a guest network feature.

All of my lights are configured on the main (non-Thread) network, working as normal.

I purchased an ONVIS HS2 switch (which requires a Thread-capable hub) to control those lights and scenes, and the MiniPod to act as the CONNECTED Home hub (with Thread).

On my iPhone Home app, I create a 2nd home (call it ThreadHome). I switch my iPhone over to the guest network and bring up the MiniPod from scratch. Since the miniPod is the only "Home Hub" in the ThreadHome on its own local network, it defaults as the "CONNECTED Home Hub" and is not interfered with HomeKit communications on the main network. The ONVIS HS2 operates Thread over BLE5.

In the ONVIS app (a programmable switch), it is configured to control the lights on the main network, either directly or through scenes.

Now, Thread-based communications are not taken down by the OG HomePod, and it all Works as imagined.

Aha! or Ouch! One drawback... The eero guest network does not support audio streaming. It is strictly peer to peer. So the MiniPod is now just that... with no music. It still has Siri but now it is limited in the current configuration.

I suppose another workaround is if you have a 2nd wifi router, that could be set up as a separate local network with a wired upstream connection to the main router (that connects to modem/internet). This provides the needed isolation for Thread-based communications.