You know, I overlooked the conversion option completely, no wonder it felt tedious, thanks for pointing that out.Repeat stage 1 over and over for parts and crystals, go for all 10 chests so you get the +500 ether crystal bonus. Any extra parts you don't think you need, you can go into the Create/Crystallize Parts menu (its next to Poppiswap) and press Y to switch to Crystallize mode. From there you just pick the parts you don't want and turn them into ether crystals.
The other problem is playing the Tiger Tiger game, every time you win or lose, you have to start the whole thing over again, adding loading times. Also every time you take a hit, you lose treasure chest, it gets annoying after awhile and is not very fun for a mini game, is just boring to have to do just for one type of character to improve. I hope they plan to do something about it in a patch update.