Orbi RBK53 questions

1. Because my Nighthawk is struggle city to get more than 300mbps from FiOS (and it’s definitely the router), will Orbi have the same issues for devices directly plugged in?
LAN devices should speed test capable up to 900Mb. Wireless will max out at around 400-500Mb based on device support and design.

2. Even though 3 units for 1600sq is overkill, my thought on the 3rd unit is for the garage, to help broadcast to the outside and that general area of the house. I hope to daisy chain them all together via wire. Is this a concern with too many units?
1600sq ft you really only need a base router and maybe one satellite for the garage. If your going to keep the same center placement of the router. I have a 5000sq ft home and use juse the base router at one end up stairs and 1 satellite at the other end of the house with 40 feet inbetween. I get great coverage and performance. You may just want to place a satellite in the garage if you our there alot.

3. The fact that my main unit is close to center point, should a second unit be upstairs/basement in a corner? Or upstairs/basement center? I understand it’s a circular range, but is it a true sphere range or is the height a bigger drop off compared to left/right?
30 feet is recommended in between them to begin with depending upon building materials.

4. I don’t care about QOS or parental controls (future maybe), but I know these features destroy the Nighthawks performance. Would this be the same case?
Theres same features, however as long as you don't enable them or use them, you'll be fine.

5. Does anyone have this with gigabit and see any issues getting gigabit speeds through the router? My house is wired cat6.

QOS or parental controls and Traffic Meter, as long as you don't enable them or use them, you'll be fine.