Snow melt - help with heat exchanger freeze protection

I'm adding a 120 ft2 (4'x 30') concrete access ramp to my house in Ottawa, and looking to add some heat to it. My goal is just to avoid salt on the concrete, and to use the heat as supplemental to shovelling the snow. Manual start, with timed operation is the goal. Slab temperature sensor is an option, but really don't want / need a snow sensor. Designing for 160 Btu / sq ft, so 20K Btu total.

When I reviewed the documentation for some snow melt controllers like the Tekmar/Watts 654, it wasn't clear how to prevent a possible sub-zero return from the slab causing freezing at the heat exchanger. The system I'm planning would rely on the not so speedy Uponor thermal actuator, and I'd need to ensure the heat exchanger maintains a minimum temperature.

Looking for help on simple / robust way to prevent freezing problems at the heat exchanger. (Either mixing or sensors & control)

Also, while I'm looking at Tekmar 654 or Uponor 3040654 (same thing?), it is more controller than I really need. I'd like something I could mount on the wall near the front door, and it would be easier to retrofit something that ran off 24VAC.
Any other controllers out there I should look at?

System Info:

Existing modcon boiler ( 151000 Btu Reillo) for the radiant system heats the basement concrete floor (seldom used) and an indirect hot water tank. There is an unused position on the manifold, and an open position on the Uponor zone controller. Manifold has Uponor thermal actuators. The manifold is feed by a Grundfos Alpha 2 15-55, on auto adapt. Reillo is set to 110F for the radiant.

Rather than cut into the primary loop for the snow melt, my plan is to run a 3/4" loop off the existing open position on the manifold to a flat plate heat exchanger.

Designing for 160 Btu / sq ft, so 20K Btu total. The snow melt circuit design has two 1/2" loops in the ramp, 6" spacing. Each loop is just under 200 ft. (The choice of 1/2" was to help with a bend on a stair tread, off a landing midway) Flowrate 0.86 and 0.84 GPM, headloss 8ft.

Mini glycol feeder (Calefactio GMP4, Axiom DMF150, or similar). Grundfos 15-58 for the circulator. Expansion tank, air separator TBD.