Convert to mesh network - What do I need

I am wanting/needing to upgrade my home internet. My goal is to have one 2.4 GHz and one 5 GHz wireless mesh network. Currently, because of cable modem, router and extender, I have three 2.4 GHz wireless access points and three 5 GHz wireless access points. I need to add another extender and I don’t want to add a 4th set of access points.

Here is my current setup:

• Cable Modem, Netgear AC1750 Model C6300
  • 4 wired ports, 3 in use
  • 2.4 GHz and 5 GHz wireless
• Main router, Netgear Nighthawk X10 AD7200 Model R9000
  • Sits 3 feet away from the cable modem
  • 8 wired pots, 5 in use
  • 2.4 GHz and 5 GHz wireless
• Extender, Netgear EX7000, AC1900
  • Sits on other side of house
  • 4 wired ports, 2 in use
  • 2.4 GHz and 5 GHz

My goal

• I need to add another extender to reach a dead zone.
• I would like to move to a Mesh network so that roaming thru the house will automatically switch from one router/extender to another
• I need to maintain the existing wired connections
• I want to purchase the Mesh equipment without needing to replace existing equipment

Can I:

• Turn off the wireless on the cable modem and continue to connect the cable modem to the main router (wired).
• Turn off the wireless of the main router and connect the main router to the Mesh router (wired).
• Add a Mesh extender and cable from the Mesh extender to my existing extender. And, turn off the wireless of the extender.
• Will all of the above keep my existing network (the wired devices) functioning? Giving me a 2.4 GHz and 5 GHz wireless Mesh network?
• Using one additional Mesh extender, add it in the dead zone of my house.

What do I need to purchase to make the above happen? Netgear Orbi  or Netgear Nighthawk?