I am unsure about the wind thing with flex fence, my friend has it at her place in the desert in CA and it is extremely windy there. It has held up for her for at least a decade. With the pipe fence I would check on prices of materials. You can get oil pipe in west texas for free if you know where to look.
We recently had pipe and no climb mesh fence put in including 4 paddocks, maybe same or a bit more in size, cost was about the same as your $20,000 quote. Three rail pipe was much higher.
Resistivity depends on the exact formulation of the concrete and varies from 50 to 1,000 ohm-meters. Concrete conducts electricity poorly compared to metals, electricity seeks the path of least resistance
When we concrete fence pipe post in, there are several inches below the concrete into the dirt.
Guess that is not enough, needs to be feet down in there?
We do have long, deep copper ground rods here and there, but not that many per mile as I think you are talking about.
Also, the ground being dry doesn’t conduct as well as when saturated.
we used 10 ft copper clad rods on airports… you are in the never dust bowl where the soil is less conductive … question that may have ab answer if you use electric fencing …does it work well as the effectiveness of the electric fence is by the conductivity of the soil back to fence charger ground (ground’s conductivity increase as moisture content increase)
None of the posts we used were ever back filled with dirt as we we required to have a continuous mow strip of concrete under the fence … but the fence that I saw get hit with a lightening strike and the horse was knocked down was installed like yours.
The greatest concern I would have would be if the pipe fencing was connected to a steel building or abutted to a steel building… our barn is steel framed, we have the frame grounded in several places
I still have yet to decide quite what I’m going to do. But I think I am going to go for the pipe fencing. The fencing company I use is very upfront with everything. They told me who they use as their supplier. This will be a 5 rail pipe fence welded to metal posts. Just the fencing alone costs almost 10K. That is not including the install or the fact they will be removing and disposing of the old fence. Plus it includes 8 gates (I’m having gates installed everywhere, you can never have too many gates). I actually thought about just buying panels and using the old wood posts already in place but even that would cost me close to 7K I might as well as a super nice fence. The pipe is all 14 gauge I believe. If I lost my mind and decided to get buffalo, the fence would still work 
I’m obviously leaning very strongly toward it. It is just a lot of money to spend. Then I start thinking about the imaginary vet bills I could have and it’s worth it.
I had a two year old horse was severely injured in a pasture with continuous pipe fencing. However, I don’t know if it was the fence or another horse. So it might not be relevant, except to point out that horses will hurt themselves in amazing ways. I do like hot fencing for this reason though. The fence was a very heavy duty gauge, rough (not smooth at all) fencing. She smacked her ankle hard enough and it even chipped up her ankle joint. Pretty dam* depressing.
