You DON'T need more than one router. You need one router and multiple access points connected to a switch. And preferably you will get PoE (power over ethernet) access points. That way, you only run a single CAT6 cable to each access point and mount it where you want it. The power to run the access pint and the network signal for wifi both come through the cable. No needing a power outlet near the access point. No messy power wires to run.

I use Ubiquiti Unifi Access Points, but there are lots of them. You space them to be close enough for coverage but not too close to have too much overlap - both on the same floor and any floors above or below. So planning is crucial. You need a PoE switch or you can use the power bricks that come with the Unifi, but a PoE switch will result in power connections.

Read up. Come back. Ask more questions.
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You can get a free wifi analyzer app for android or iOS. When you initially set up the router, you can check what the signal strength is in various places. You'd be surprised at which areas have good coverage and which areas don't. Sometimes, there is a clear line of sight to the router but the coverage is really bad. Sometimes just going one foot forward will give an amazing signal and one foot back, no signal at all.

Phones will not see anything below -80dB. PCs will need at least -70dB.

Right - good rule: if your iphone/android phone can see and use the wifi signal, a laptop/PC should be fine. Another hint that a lot people use is to turn down the power on the 2.4GHz band to medium and leave 5GHz on high power. That way a client should favor the 5GHz band. There is also "band steering" on the access points to push clients to favor the 5GHz band.

You might want to buy one access point or even a dual band router (or use your core router) and move it various places to get an idea of where access points should be. The site survey is critical to the end result.

You might want to use one of those devices that sendes the internet signal through your powerline. That's generally better that trying to extend your routers range.

No. Those are non-professional "solutions" not meant for many users over a large coverage area.