Just letting people know, starting towards the end of EP5, you start depending less on RNG and as long you grind EQ/UQ and get stone/material drops you can "build" weapons and units. This is especially true in EP6 - we just got our last EP6 story boss EQ, and the 15 star weapons from it you can build with the stone that the EQ drops, emerald (crafting mat), Plamiaxxx materials (crafting mat, 1 tier higher than Goldxxx), excubes, Divide Quest fragment drops, and ultimate boosters. The upgraded version for it though you are going to need more rarer materials.
As long you don't mind grinding for more 15 star weapons, there is always things you can do on PSO2 JP, don't really have content drought like on global right now. Someone mentioned collection files for current EP4 EQs, I think they didn't add it in because unlike JP, they are going to be irrelevant very fast - we literally had months in between each EQ boss on JP to grind 6 copies (to get it +35) of the weapons we wanted - cause the greed sensor never drops the weapon you want.