But I have also been meaning to try out various allocations such as a first partition that is less than 128GiB and subsequent partition(s) larger than 128GiB. I now hope/intend to try it sometime soon, and I will report back with how it goes. But I would be happy if someone beat me to it... ;-)
Happy to report that I've extended that last partition to ~220GB to fill the rest of the card, and it seems to work fine! Files will still read from it just fine (though I have nowhere near the ~400GB of music needed to properly test that largest partition. (That first partition is still less than 128GB by the way!)
I now have a 512GB iPod 6th gen that works fully with Rockbox! Thank you everyone for your help!