Well I said rotate them all with events so that we'd end up removing the rareness factor almost entirely. Lets say that each individual weapon had its own set of "changers" for each possible permutation of change, and that three changers were available at any one time each for 6 days such that every 2 days a changer was swapped out.

5* Weapons: ~18 swords, ~30 handguns, ~19 bombs = ~67 total
Possible attributes: 6 status effects, 4 damage types, 4 modifiers (MSI, ASI , CTR, damage) = 14
Total changers: 14 * 67 = 938
Total time to rotate through all of them at 2 days per changer: 1,876 days, or a little over five years.

Are they rare? Not at all. There would literally always be some within your grasp for 50K crowns or so. Would it be hard to get the one you wanted? Absolutely. But say you want fire for stagger storm and shadow damage for final flourish comes around... there's nothing to stop you from wanting that too, and there are a TON of intriguing weapon combinations to go around.

Nor would it be plausible to even say what was the best set of attributes. There would almost certainly be popular choices that people wanted (like pure damage combuster for FSC) but no one would be able to collect enough to make an exact call when it comes down to it. How would a pure piercing voltedge that dealt very low damage that dealt stun+poison and made you walk faster fare compared to a piercing glacius that made you walk slower, deal more damage, and freeze at a lower rate? I couldn't tell you if I wanted to beyond speculation.