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I did a lot of pipe thawing back when we had the shop in the mountains
Infiltration was the biggest cause of freeze up. Windy days at below freezing conditions, we were always frozen pipe days. That’s when I got sold on spray foam, even a 1/2” layer with batts over it would seal a building against infiltration.

Underground piping froze when we had years with little snowfall to insulate the yards

The seal around the bottom of the wall and the plate and the floor is usually poor. pipes usually freeze not when it is the coldest but when it is fairly cold and there is a wind to push air in through openings in the building that are close to a pipe.

How much work are you doing and how much piping is in the exterior wall? Perhaps moving it will solve numerous problems. Glycol needs maintenance of the inhibitor. Placing the insulation between the exterior and the piping would seem to be an amount of work where re-routing the piping would be a negligible additional step.

hot water does NOT freeze quicker!

I would think that if the wall bay insulation is placed between the outside wall surface and the piping, then the piping would remain at or near the indoor ambient temperature, so it would not be much more vulnerable to freezing than any other interior piping.

In general, the efficiency hit of glycol is way overstated. It doesn't make your burner burn any less efficiently. What it means is that slightly more fluid needs to be pumped to get the same heat transfer. Your system will have a slightly lower maximum output, but unless it's undersized that shouldn't be an issue. The circulator pump will consume slightly more energy having to pump more fluid but that's the extent of the energy penalty.

It is an ongoing fallacy that hot water freezes faster than cold water. Personally, I believe the fallacy is developed from the fact that hot water cools faster than cold water. People without any intelligence then extrapolate that hot water must freeze faster because its temperature is falling faster. Nothing could be further from the truth.

With regard to piping in the exterior walls, I restored one house to get the iron piping out of the living space. The look is less than desirable.

Since the wall was open, I utilized 2" styrofoam, 14.5" width for the entire height of the bay. The styrofoam was cut slightly large so it was pressing on the studs without any gaps. The copper lives in the 1.5" space between the sheetrock and the styrofoam. The copper is not insulated and the intention is for the line to warm the fairly small space left in the bay.

The system works perfectly down to -3F. However, it has never suffered a power outage at that temperature to verify that it would be successful under those conditions. I highly doubt that even interior piping will survive more than 24 hours in the event of an extended power outage at -3F ambient.

I believe that is a Verticell indirect water heater