I find this frustrating as well. I have run out of room for all the different forms. There are over 800 base Pokemon, plus nearly 100 with gender differences, 30 unown, 20 Vivillon, 6 Rotoms, 7 Miniors, 5 versions of Flabebe, Floette and Florges, 4 versions of Deerling and Sawsbuck, 3 versions of Burmy and Wormadam, Different Shellos, Gastrradon Frillish and Jellificent, 23 additional alolan variants (not counting ones that have their own number, 6 totem pokemon per game, multiple hat-wearing Pikachu; and that's not even counting storing the different forms of pokemon like Deoxys and Tornadus, Thundurus, Landourus etc. And then shibies is a whole other situation...
I would like to have had a complete collection on the one cartridge, but even with storing some on Poke Pelago it isn't possible to fit them all.
I hace Bank, but I personally prefer to have my collection with me, rather than on a server.