I’ve noticed some manufacturers, namely the Eero now supports Thread in their router. For the sake of simplicity, it seems like a good idea that all your wireless radios live in the same box (which is probably well placed in your home for wireless reception). However, given that thread runs in the 2.4Ghz space, do you think it would actually be preferable to keep these separate e.g a Connect ZBT-1 connected to your HA box with a USB extension, some distance away from your existing wireless AP?
I think you’re right, simplicity is key. It makes sense for the vast majority to use the Thread network that comes with their router (or HomePod, Echo, etc). In theory, the device can automatically adjust WiFi and/or Thread channels to substantially avoid interference between protocols. The caveat (today) is that HA integration (i.e. credential sharing and commissioning) is a work in progress, and still suffers from some UX issues. This continues to improve over time, and the recent finalization of Thread 1.4 standard should make it even easier (eventually).
Setting up your own OTBR using ZBT-1 or similar is a great option for tinkerers, open-source or privacy advocates (or general opponents of the tech-industrial complex). Its main advantage (today) is that HA integrates cleanly, but comes with a cost of increased setup and maintenance complexity. I don’t think it offers any RF advantage (the reason an extension cable is recommended is due to usb3 port interference, not WiFi proximity), and passes the burden to you to manage the channels to limit interference.
Also recall Thread is a mesh, and for under $5 you can build your own repeater, powered by any USB port in the house, to extend coverage if your Border Router location isn’t ideal. I built a repeater in five minutes using espressif zerocode firmware which commissioned over Matter to my Apple Thread mesh.
A final note that all of your 2.4Ghz devices will work best when you keep high-traffic sources (namely video streaming) exclusively on >5Ghz bands (if you must use 2.4Ghz cameras, avoid 24/7 recording and stream only on demand). Most IoT devices generate very little traffic and many dozen (I have about 50) should peacefully coexist on a well-designed 2.4Ghz network without interfering with Bluetooth or Thread.