I've been finding that tuning any accordion between low G and B is such a pain. I'd even prefer to tune a D accordion (and anything up) over a C.

The pitch vs. pressure factor is tricky on the low ones. Finding the equilibrium is difficult, and you have to tune to near the amount of force you particularly like to play.

I guess it all comes down to, lower reeds respond slower, and are more sensitive to the amount of pressure, while you're tuning and playing. Especially with shallow reed chambers (which I prefer, when it comes to air usage).

I ended up with a lot of trial and error with my low G's bassoon set.

It is definitely an art form.

Just experienced the same with my G, any tips? - Nout

The issue is it takes a while for the big reeds to stabilize, especially when it's the only one sounding, so you have to provide plenty of bellows travel to give the reed a chance to stabilize to playing pressure. One thing I do is tune the small base reed first, then the big reed, then sound with the smallest reed and biggest reed together. This allows the big reed to stabilize better, and both your ear and and a strobe tuner can pick up the difference, if there is still one.

Yessir that's how I do it