I dont know what I was doing last night, but putting it all into ecalc again with
- U7 v2 420kv
- 16x4.5 T-Motor CF polished props
- 12Ahr 6S
- 6kg take off weight
gets 16 minutes flight time and 3:1 thrust to weight ratio.
You can go up to 9kg take off weight with that combination and get 9 minutes and 2:1 thrust to weight.
Changing to 18x6.1 props gives 1 extra minute flight time with the payload and 2.6:1 but you are starting to get near electrical load problems. This combo could be OK if you always have the payload attached, but it would be a bit over-powered and possibly very hard to tune without the payload.
You wont be able to use a cheap low C battery pack with this.
This is the range of props I would use with those motors.
https://store.tmotor.com/category-68-b0-Polish.html
or you could go with the folding props for slightly reduced thrust and a bit more convenience.