I personnaly doesn't mind doing the first kill of a Raid boss solo and taking between 10-20 minutes depending on the character and various factors.
But as I always convert all my chests in scale chests and put glowcap everywhere, I need to kill Dragonfly and Toadstool a lot. And in the long run -the short run actually) it isn't fun. I have more fun fighting Goose/Moose because its pattern is random.
I also think that abusing the deconstructing staff to duplicate both scales and skin is boring, and needs anyway to kill Fuelweaver to regen the ruins. Not fun, same problem.
I thought recently, why can't we have a system like if you already kill a Raid Boss, the next encounter will be shorter ? Per exemple, constructing a statue of the said boss in its surrounding at the beginning of the fight will alleviate the boss of a % to its HP. This way, the first fight is still legendary, the other ones (you already know how to deal with the boss) are just shorter.
You know, I just don't like to spend 10-20 minutes killing the same boss that is not a threat since 3 years, every 160 minutes. Make it 5-10 minutes please.