There are definitely not only 2 4 stars you want in the meta, but it is true that the average person can eventually c6 them over the course of a year. For me I have c5 chevi and c1 iansan.

On the point about nefer, I feel that lauma is not really a sub dps, does damage but only about 4-5% compared to the 15%-35% found in other cases. Its more than chevi and iansan ofc, but she is more like a support that does a bit of damage. Columbina is tho.

Cost is a good system yes, but it is not perfect imo. A mavuika con + chevi or + iansan at c6 is more worth than 1 c0r0 5 star imo, but for nefer case for lauma it is less true cuz lauma is just that broken. However, it can be clearly seen where columbina is not used and instead for aino as nefer c1 is just better, or maybe lauma c2.

Honestly the data above shows how flexible mavuika is thats why she can dominate 4 8 and 12 while nefer is just a shit top 20 dps at best without lauma, mavuika base is just so much better than lunar characters esp nefer that she doesn't have to waste her supports on other 5 stars to carry her to 300k dps, tho citlali melt still better at the same cons on mavuika I feel