Pokémon Bank does not reward you for completing its National Dex at all, meaning there is no real reward you get for filling in the National Dex all the way up to Gen 7.
The closest thing to an award would be the ability to speedrun for an indefinite amount of Magearna-O in Pokémon HOME (by linking Bank with HOME then pumping Pokémon in the GTS for all the Galar Dex Pokémon, then deleting your HOME account after getting Magearna-O to do it all over again), but as someone who enjoys Gen 7 and have no real desire to get Sword or Shield, it sucks how you cannot transfer Magearna-O back to the Gen 7 games or even get Magearna-O off Bank for Natdex Completion.
What I did for this case was complete the HOME National Dex, all the way up to 905 (only 1-890 is required for Mag-O), then when I got Magearna-O, there very coincidentally became an update that allowed anyone to delete their HOME save file the day after. I decided I was going to delete my HOME save file and use an external device to create a Magearna-O in Pokémon Ultra Moon, then consider it as "transferring the Pokémon HOME Magearna-O backwards".
Am I crazy? Yes. I do not care however. I'm just playing Pokémon how I feel I'd enjoy it most
.
I then created a new HOME account, linked it with Pokémon Bank, instantly got 1-807 registered, filled the HOME dex up to 890 again for another Magearna-O, and gave that one away via Wi-Fi room giveaway last Christmas.
Note, I decided to sacrifice a lot of valuable Pokémon I had in the Gen 6 (and to some extent, 7) games in order to get the Galarian Pokémon from the GTS, so anyone doing the same would need to have the resources (legendaries, fully evolved Pokémon, etc) to offer up for trades.