I’d suggest playing with temp differential if possible. There’s pros and cons to high vs low, it really depends on your priorities. Noise vs energy efficiency vs equipment stress. To me the old standard of 1F is excessively tight. Heating wise I’m ok with 2F, which means it starts/stops a lot less.

As far fan noise, that’s a trade off too. In my experience your brain ignores steady sounds pretty easily. I forgot earlier I had the dryer running until the cycle ended. That hum is just something we filter out when we get used to it. On/off however is jolting. You don’t really adjust to that. Modern better systems are obviously quieter than those hotel units but never silent. That’s a lot of air moving through a small space. Physics comes into play. Central heating with multiple registers each with less airflow will be quieter. But still not silent. You still hear it turn on/off. But less airflow makes it less obvious.

I don’t think there’s hard rights/wrongs here. It’s just what you prioritize in what situations. My bedroom I care more about noise, so I overcool it at night when it’s time for bed, then have a higher differential and low fan speed. That lets me keep it cool in the summer and not turn on so much. In the living room I let the thermostat control the fan speed so it will be low if possible (maintaining temp mostly). I care less about noise.

So you can't increase the boiler temperature without risking scalding your tenants? I imagine there would be a way to increase the heating water temperature separate to the house hot water, even on a combined system.

My house was struggling to get to 65 with the boiler set to the default 180 degrees, I increased to 210 and things got much better.

Robin.