Adding Collection Button

Not a bad idea for the time being, but probably instead of a button a new tab in the main menu would be better. Then once you select it would show the current deck editor without the "editor" part, just the left part enlarged.

The ideal tho would be to make it better without stack the rarities, better filters and more sort option, etc. basically similar to TCG Online Collection feature, but those bigger changes could be done later. Your suggestion for now would be smaller but an user experience improvement and more intuitive for sure.