looking for a keyboard where one can change Voices with one key rather than setting voices in Banks & Registrations.
Can you be more specific? Do you literally want to select a sound with a key rather than a front panel button? There are boards that do that, but it's generally not preferable because you have to stop playing to change sounds.
Banks typically come into play when boards have too many sounds to easily manage at once. Boards with few sounds probably don't have banks. Registrations (or an equivalent) can be required when you want to select, not a particular factory sound, but rather a personally customized sound or combination of sounds that you've created e.g. via editing, splits, and/or layers... but if you're just playing factory supplied individual voices, you shouldn't need to bother with registrations even if a board supports them. But things can be handled a little differently on a particular board.
But maybe we're just getting hung up on terminology. Can you provdie an example to show exactly what it is you want to be able to do? (And perhaps an example of a board that didn't let you do it?)