The simplest solution to all of this would be to move the goalposts for synthesis and make the premium ammo equal to the ammunition we buy in the stations, with the others being weaker.
For example, rather than "basic", "standard" and "premium" synthesis, we have "backup", "makeshift" and "basic" ammunition available in synthesis. The top-tier basic synthesis would be as good as station resupplied ammunition, with the others being 30% and 15% weaker respectively.
Although personally, I feel that synthesis in general needs a lot of reworking as it makes things like ammunition limitations completely void in any kind of competitive scenario as well as rendering exploration even more of a doddle than it already is. The only synthesis that I am happy with in its current state is the FSD injection. I'd be much happier if they moved synthesis into an actual module that could be loaded up with a certain number of recipes, with premium synthesis taking up multiple recipe slots, ideally with changing all of the material requirements into commodity requirements.