I have a 1500mbps line.

Phase 1
Installed 3-set WiFi 6 x1500 Deco home mesh. 
Main is wired to router as Access Point, giving WiFi +600mbps. This is same as WiFi from router. So no loss there. 
Satellite 1 connected via WiFi connected via WiFi, was up the staircase 1 floor from from Main, with floor and wall between, about 10-15m distance.   Sat 1 gave WiFi +/- 100mbps

Satellite 2 connected via WiFi also on floor above near bedrooms, straight above the Main and 7m straight line from Sat 1, with no walls between. Sat 2 gave WiFi +/- 60mbps, but no more than 30 in rooms. It also connected more to Main than Sat 1

This was not working. 
 

Phase 2

Added more satellites and rearranged... 

Main is wired to router as Access Point, giving WiFi +600mbps. This is same as WiFi from router. So no loss there. 

Satellite 1 connected via WiFi is now on same floor about 7m away from Main, and while there is a wall between, the rooms are connected by double glass door. Sat 1 gives now WiFi +/- 300mbps

Satellite 2 connected via WiFi is up the staircase 1 floor from Sat 1, no real walls between. Distance about 7m. Sat 2 gives WiFi +/- 150mbps. 
Satellite 3 connected via WiFi down the end of corridor (leading to all room) from Sat 2, about 10m distance no walls between.  Top speed I get in rooms is 100mbps. 

I tried to add satellites in between, to shorten distances and reduce obstacles, but does not help. Actually feels it is slower. 

Some facts:

Main seems to cover the whole ground floor with good speed, no problem. 
regardless of how many sats I put on second floor, I don't get beyond 150mbps at the best point. Mostly it's below 100mbps. 
I do not have Ethernet backhaul option. 
 

Questions 

1) A better set up suggestion? I have 6 units to play with. 

2) Can adding more satellites reduce distance and obstacles, but the increased hopping offset benefits? 
3) Should I all devices to choose their source or assign the map in logical order?

4) Are coverage overlaps bad? 
5) What speed should I expect from the furthest satellites? I really hoped to keep 300mbps throughout. 
 

Thank you for any tips. 
 

sorry for long message, but wanted to provide full picture