I am a layman myself but I will try to answer some of your questions.
Just to clarify before I explain things here, a wireless router consists of three things:
-First, a router that controls the traffic between its own network and the infinite networks that exist.
-Secondly, a netword switch, which controls the traffic between your own devices connected to your network.
-And thirdly, a wireless access point (AP for short), which makes the network wireless and connects wifi enabled devices to the network.
Question 1: This is called triband, this is built in especially in the newer Wifi 6 mesh systems. This allows the different AP's (Access Points) to communicate with each other without affecting the other two bands.
A band can be thought of as a lane on the motorway. Devices are vehicles travelling in these lanes. If you have only two lanes, the different Ap's have to communicate over these lanes and slow down the whole traffic, but can also be slowed down by e.g. a slow Tracktor (an older device).
If you now add another lane that only serves for the communication of the different APs, then they have their peace and can always work undisturbed while all other devices are driving on the other two lanes.
That is why such routers use triband so that they can guarantee the high speeds of the Wlan 6.
2nd question: Such devices exist, but they are less and less built for "normal" houses, because many prefer to have just one connection to the network and the other two ap's can be distributed where they want.
I found these routers, but I'm not sure if they have all the ports like on the first picture.
ASUS ZenWiFi XT8 WiFi 6 AX6600 (2 Pack)
Question 3: Yup every system has a weak point. If the main router fails then nothing works. There are probably expensive routers that you can run redundantly, but for a private network at home I find that almost excessive. But if you're looking for something like this, I recommend you get advice from professionals who can also install such a system at home.
Now what you are looking for:
I've searched around a bit myself and haven't found anything that exactly meets your requirements. There will always be the weak point that if the main router fails the whole system is down.
One possibility would be Ubiquiti
https://www.digitec.ch/de...
You can buy everything separately, i.e. router, a network switch and also several Ap's to build up your own system.
But even that will have a weak point.
I hope I could be helpful and have fun building your network :)