https://blackmtncycles.com/frames/mcd-frames/ $700
8/5/8 butted heat treated tubing
thru axle
clearance for even larger than 45mm tires
mounts for front and rear rack, plus fenders, plus 3 bottle mounts
It doesnt have flatmount brakes and is a steel fork.
I am guessing you are across the pond, based on considering Kinesis and the Sonder?
The Jamis frameset for $850 would be tough to beat if the largest size fits you as it has everything you want...except the price.