Thermostatic mixing valve

It may not be necessary. What type of emitters does the system have? If it has low mass and high temp emitters like fin tube or convectors or hydroair coils it likely will keep high enough return water temps to not require return water temp protection. If it is cast iron radiators it may need protection although with the small piping sized for a pumped system it probably will get up to temp quickly enough to not require return temp protection.

If the setback causes it to run such short cycles that the return water never gets up to temp then it will require protection.

if you have all high temperature emitters, really no need for a mix valve

You could run the boiler on outdoor reset, maybe 150- 180 range

That increases efficiency and often comfort as the boiler cycles to the zones less

I would watch the boiler operation from a cold start condition

Ideally the return hits 130 within 10 minutes of run time

If the boiler is oversized, you tend to see short inefficient cycling

If the system has a lot of large steel piping, lots of water, it may need some return temperature protection

So gather some runtime data to see if return protection is needed.

you could potentially slow the fans down or increase the water flow if the return isn't getting hot fast enough. or increase the cycle length on the thermostat if the call is satisfied before the return gets hot.

If you need that full boiler output you are looking at moving around 48 gpm. That would take a sizable thermostatic mix valve, $$. Fixed temperature thermostatic mix valves from Caleffi or Danfoss specific to boiler protection go up to 1-1/4" 16 Cv. Too small for your application.

One option is a 4 way motorized mix valve. A 2" tekmar valve has a 60 Cv, which means with a 60 gpm flow you have a 1 psi drop across the valve. I think the 1-1/2 is cutting it too close.

The advantage to this valve is it protects the boiler but also allows you to modulate the temperature to the heat emitters based on an outdoor reset function.

A 4 way valve give you two mixing points, one protects the boiler return, the other blends the supply based on outdoor temperature.

A link to tekmar Info below on control settings and piping.

https://www.watts.com/dfsmedia/0533dbba17714b1ab581ab07a4cbb521/81322-source%7D

A variable speed pump method looks like this, but it would take an additional pump able to move 45- 48 gpm, the 4 way may be less $$.

In your case you would need a boiler circulator, the variable speed "crossover" circulator, and the pump(s) for distribution.

This schematic shows a buffer tank as a load, so just two circulators. You still need a system pump or pumps