Replacement Thermostat Question

I currently have a "newer" Honeywell round thermostat that I believe has issues. The switches are set properly. For example, with the room temperature showing about 66 degrees, I have to set the thermostat between 72-74 in order to turn on the boiler. If I try to reduce it to 70, as soon as it goes below the 72-74 range it kicks off. If I keep it at 72-74,it will run and maybe bring the system up to 68-70. If I set the temperature to 78 and it brings the system up to around 72.

In a replacement, I was going to see if a could find a new old stock Honeywell T86A (merc), or something similar to get better/more accurate performance. I had to search for a vintage style thermostat a few years ago at my old house to control my pellet stove as newer ones didn't seem to have a good enough "closure" to simulate a short circuit, allowing it to fire up.