Flash modding iPods is a cool use case for larger-capacity SD cards. However, the limiting factors seem to be the database file for the songs on the device and the RAM available.
At a certain point you get diminishing returns on the card capacity as you couldn’t fill up the card with songs and have them all be indexed without the iPod crashing. In these situations, one can be fine staying at 128 or 256 GB.
I mean if they don’t use it for anything else it’s just a funky food bowl for the cats