Was in a similar boat to you, had about 10 routers doing all sorts of things.
However the wifi was always sub optimal.
Ultimately I ran ethernet in my house, ditching the routers and buying a “proper” access point.
If you are prepared to run some cables, it makes sense to buy a managed switch and an AP. Then you can run whatever vlans you want. Better APs will have about 8 SSiDs that can be used which can map to your vlans.
I’ve not implemented a DMZ but always thought they were intended for things like webservers you hosted yourself, don’t think having your IoT devices there makes sense.
Honestly, buy a managed PoE switch and a couple of access points, then give your routers away. You will always get better coverage with an AP because it’s much easier to place than a router acting as an access point.