Ya'll enjoy yourselves down there! Take it from ben-there-done-that, you are only young once and responsibilities tend to increase later in life which can impose limitations. Make the most of all years but especially the next ten years. Salud.

Do you ever get wind in the area? Usually formative pruning would start about hip height. Looks like the tree spent a lot of time in a nursery crowded up with other stuff.

It should cover the fence easily. What you could do is to prune it back annually to remove the old 'thatch' and let it start over. Mine seemed to respond to the cold-then-warmer weather this past month with fresh growth even with no care so it must be a seasonal 'flush' that is setting up. So, a mid-winter pruning might keep it tidy. A single wire might be best, it scrambles very well and mine have been able to grasp the bark of pine trees and begin making their way up. Using a wire grid might just make it harder to prune. 20 feet isn't very long, it will probably make it to the end and come back again.

OK, now realize pitaya has very shallow roots that spread wide and far.  Here's my plan.  Thoughts are welcome.   Gonna take a long strip of 16" RootBuilder, cut it in half making it 8" tall.  Wrap and tie it into a large rectangle about 8' long X 18" wide squaring off the corners and place it against a greenhouse wall.  Backfill with a very sandy/vermiculite soil with some humus and LAY the cuttings down so that 1 side is just below the surface for rooting.  Done this with cacti, why not pitaya?   Should give me a lot of output along the cutting.  I can plant 3 different available varieties in this one long pot.

It will be interesting to compare pot and bed culture, you could be breaking new ground.

Hold the cuttings in a cool shady place till they get good and dry. Large growers just stand them up leaning but touching soil until they strike roots.  After that, plant in your beds. Some let cuttings heal and plant in pots or directly in soil, I did that.
https://www.youtube.com/watch?v=rIx7tJrSPUc&t=3s

You want single growing stems going up with no branching, you should continually remove branches when they start and into the life of the plant. Train upwards to top of trellis and a little beyond, maybe five segments beyond then tip prune to encourage top branching. I have seen cattle panel trellis in a greenhouse used before.

You could try laying them down on an experimental basis.  But what you want is a very vigorous SINGLE stem growing upwards.
The plant seems to respond naturally to a single stem with fastest growth, and the goal is to get up as quickly as possible. If it slows down or stops for whatever reason you will have to wait for a new tip branch to form and accelerate upwards again. A good grower might never stop. Then when it reaches the top branch out and make the drooping form. Once those branches get full growth you are set to flower. With luck starting now that could be in early fall, about 8 months. Not sure what they will do during Fall in Texas, there may be some day length sensitivity.

I'm just getting started, and what I am telling you is mostly gleaned from watching what the professionals in Vietnam have done, they lead the world in production so I am trying to emulate them. Others may have new and different methods. Mine are just beginning to branch at the top. In my area established plants have come out of dormancy and are actively growing. Flowering here should be starting early summer.

How do you know when possum purple is ready? with panama red, as soon as the green fruit started showing some red, they were good to eat. These are purple from the time they set fruit, so not sure what kind of change I should be looking for. Do I have to wait till they start to shrivel?

I wait for fruit drop

50 lbs per tree sprinkled under the drip line wouldn't be too much for a mature tree. It generally dissolves into calcium and sulphur.

I have heard exactly the same dieback-after-pruning story from another grower Berto here in Fort Myers. My one year old emerged from dormancy with very few twigs dry and pruning that small amount caused no problem.

The only worry point is that the crotch angle between those two branches that were grafted together is very acute and will be a weak point in future winds -- pretty easy for one of those limbs to tear right off through the fused cambium.

In researching tree pruning I also wondered about crotch angles. One reference showed the result of a close crotch angle this way:




A analogy in engineering would be a bar or plate of metal with a notch in it. Compression stress builds up at the notched point when the metal is bent and failure will occur at that point.

I recently ran across what appears to be a new archive of the Proceedings of the Florida State Horticultural Society.
It has a fairly good search feature which you can use to research topics of interest. The information goes back nearly 100 years.

Have fun!
http://journals.fcla.edu/fshs/issue/archive

Cutlass or even Cut-lash is a common name for it in the English speaking Caribbean islands.

I just bottled 1-1/2 cases of Caramabola wine. It was the strangest wine to make but came out fine. I started it in September before Hurricane Irma and it bubbled for over 4 months with champagne yeast. I was afraid to bottle it till now, six months later.
I've now got 1-1/2 case of Passionfruit wine settling in the secondary. So this year made Mombin, Mango, and the other two.

Seven feet is a standing broad jump for deer. You need 8 feet. A big cost is the posts use the plastic pipe extensions it works.
https://www.youtube.com/watch?v=7gi7aD3jWnw

I've done well since my wife made friends with the produce cutter at the grocery store. Some weeks I get 12 or more tops. Half of these have been too mistreated or are too old to grow, but over the year I've been able to get over a hundred. I'm thinking of getting into something different from the Gold Doles, and can order from agristarts. The 1955 division method sounds interesting and I may eventually sacrifice one to see what happens.