Bosch Stainless steel Condensing Boiler

Nothing weird about that gas valve. It's a zero governor gas valve and operates in a vacuum like most mod/con boilers. Adjusting it is right in front of your face but you need a combustion analyzer to properly adjust the valve.

Have you performed preventive maintenance on the boiler? That's where I would start if you don't know when its was last pm'd

You don't use a manometer to set these gas valves. You need a combustion analyzer. You can use your manometer to check incoming gas pressure to the gas valve to make sure you're not losing gas pressure. I would not recommend any adjustment without analyzer.