Help Setting up Two Parallel WiFi Networks

Thanks to both of you for comprehensive responses!

I had already ordered the Eero system, which has now arrived, and I have set it up, in parallel with the Orbi system (keeping the IoT devices on the Orbi system and using the Eero system for media and everyday internet usage). As I hoped/expected, it delivers much faster wireless speed, and I don't seem to run into much trouble with interference. The only downside is that for some devices (Apple TV, HomePods) I need to be on the same network as those devices for controls to work fully, so some manual WiFi switching is required.

I guess there is now way to establish some kind of local connection between the two networks that avoids this (so that devices on respectively 192.168.x.n and 192.168.y.n) can talk to one another directly?

However, from what you say about Orbi and number of devices, it seems I should be able to make everything work using only the Orbi system. You suggest some tweaking I have not tried (and someone else suggested wiping them and rebuilding from scratch). I guess I will have to try that some day when I have a lot of time, but I deeply fear breaking things that have taken very long to set up, with IoT devices whose static ip-addresses allow Homebridge access, port forwarding to numerous Synology NAS prosesses, etc.).