Bigger caliber doesn't matter for SP artillery with automatic loading.
Rate of fire for larger guns isn't all that slower, as it's loading limited(thermal limitations for the barrel are quite far away). Historical 8" automatic guns exceeded 14 RPM cyclical, i.e. higher than almost all modern SP guns.
2S7 shoots and scoots quite fine, basically like any similar gun.
And if any SPG is spotted by an UAV, it's vulnerable to attack.
6" artillery is closely tied to human loading and logistics. As soon as it isn't the issue, it's fallacy.
Small to medium artillery, per Ukraine, are indeed good, but it doesn't mean that the trend is downwards.
What's bad is single 6" solution. It is neither strong enough(it's reasonably counterable by field forrifications and heavy armor) for many uses, and 6" shells are still damn hard to manufacture at war scale.
6" unification made sense for late 20th century mechanized warfare - preferably in a short, intensive conflict, leveraging prewar stocks.