Simple outdoor radiant heat attempt

What I’m thinking about here is a hydronic radiant system for my outdoor patio. It appears fairly straight forward. I’m a semi-retired engineer and have done virtually everything on my home, from framing, plumbing (replaced all copper and waste runs), all the way through vent sizing for remote bathroom fans. Some years ago I did install my first (any only) radiant system during our master bath remodel. It was an electric system that I’ve been very pleased with. Given that it was electric, this was easy to do. There is nothing like stepping out of the shower onto warm tiled floors!

We are in Southern California near the coast so it gets chilly in the evening from the ocean breeze (or perhaps that's due to my age . Many folks here use a propane radiant tower (similar to those in restaurants) but I’m not a fan of that solution. Because we’ve decided to enlarge the patio to about 330 sq. ft. I now have the opportunity to add a pex line before hiring someone to pore the concrete. Originally I planned for a 500 ft. loop of 0.5” Pex-AL-Pex for installation simplicity but now I’m revising that to just oxygen barrier pex to allow use of easier compression fittings. I’m also researching whether reducing this run to just 300’ would slow the warming rate too much. After doing insulation research I’ve decided to use FORMULAR 250 insulation (the corning site recommends FORMULAR 400 for under slab use but that seems overkill) over the existing tamped sandy soil. I know 2” would be ideal but I may reduce that to 1.5”. I’m not too interested in efficiency or BTU sizing as this is to be used outdoors and it will only be used infrequently (i.e. social occasions with guests or family outdoor dinners).

Now comes the interesting part. I’d like to use my existing AO Smith Vertex 100 gas water heater (GDHE-50) which sits about 100’ ft from the patio. This is set at 130 deg. One thought is to run a ¾ in tap with an added check valve from the nearby (~12 ft) kitchen hot water pipe to the radiant heat run inlet. Then I attach a temperature and pressure (T&P) relief valve to the radiant heat run outlet that places the valve outside next to the patio somewhere. This would allow me to manually flush the pex run until I know it is charged with hot water.
However, it’s likely this is not going to be sufficient for adequate warmth and not practical. Therefore, the next thought is to add a variable speed Stainless Steel circulator just before the radiant inlet with a dedicated pex return line from the radiant outlet to the water heater. The water heater even has a fitting for such a return line that I’m presently not using!

I should note that I essentially have something similar to this in two places already. A decade ago I installed small residential “under-sink” recirculation pumps in the kitchen and master bath. These pumps are designed to send water back through the cold line to keep water hot at the fixture. I found these were needed since both kitchen and bath are ~90’ from the water heater and my piping runs through an open crawl space that significantly cools any stationary hot water. Without them we wasted a lot of water and it would take forever to get any hot water. To ensure silent operation I installed Sioux water hammer arresters in the piping leading to and from these pumps. The disadvantage of the recirculation system is that cold water is never really cold, it’s lukewarm.

In the case of adding a circulator to the radiant heat loop, I’m thinking a dedicated return line is necessary due to the greater demand and greater circulator pressure. Given it’s pex, a simple 100’ return run is relatively easy to do. This also insures the water return goes directly to the heater for disinfecting.
A third variation is to combine the two with the addition of a Y fitting and another check valve. This would allow me to flush the pex radiant line before a planned use and then switch on the circulator to hold the temperature via is predetermined delta T controller with a thermostat demand setting. Of course this would also allow use without flushing the line first as well.

I appreciate any thoughts on this plan from the many experts here.