You might not even need that. From a quick glance, the entire Sega Megadrive collection (not counting CDs) is 1gb or so. A full set of GBA roms is about 2-4gb. Older consoles you're looking at less than 200mb each. You could likely get all the games you want on a cheap 32gb sd card with room to spare. If you really want extra space, you can likely half the sizes of those collections by getting rid of duplicates and junk you'll never play.
CD based consoles are another matter. But even then, 256gb USB flash drives are tiny these days, and fairly cheap too. Something like this cost me around £25 not too long ago.
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You could get about 100-200 CD games on it and all your 2D consoles no problem. As long as you're not doing something ridiculous like trying to carry the entire 100% complete Playstation collection, you'll never have to swap it.