The weight or pressure doesn't change appreciably in a workshop environment which is usually 15-20 deg. C. obviously it does, which is why there is always a tolerance on the system charge label - anything from +/- 10g to +/-50g dependent upon the size of the system and the refrigerant. R134A has a very different profile to R1234YF which in turn has a very different profile to R410. If you look at practical usage, charging is always done by weight - even for fixed installations which of course will have a wider range of temperature variance, the key point is, that the system, whichever it is, has a correspondingly wide, if not wider, operating temperature range which makes the small variances in a closed system meaningless in the scheme of things.
Most HVAC systems in vehicles since the early 90's (some earlier) work on the reheat principle, that is, incoming or recirculated air is first chilled & dried before being sent through or round the heat exchanger then blended to meet the cabin heating (or cooling demand) what this means is that the aircon is operating continuously, with a device called a thermostatic valve that acts as a flow restriction to compensate for extremes of temperature so maintaining correct operation.
I'm not disagreeing with you, just saying that in the context of a small vehicle borne system, the differences are negligible.