I'm not sure keeping Sakura not bloomed is a good idea. +15% ATK +35% ATK on bosses is not that great and you will have more power with her fully evolved, even if she is too selfish. Or use Cactus for even more Crit buff. But I guess it's not a problem as long as it works.
Gold and upgrade material come easily. It's hard really early but once you got a solid base team and reduce a bit the upgrade, il will increase really fast and you will swim in it in no time.
The Infinite Castle is really fun. I think it's my favorite stuff. Even more on the hard side when you try to get 10k points in every floor.

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I'm done too with the new nations maps. It was easier than expected. Once you cleared one mission, you cleared all the missions. Only the final boss change. I guess using the big guns (litteraly) helps a lot. That was fun anyway.