FWIW my new 3+ seemed quite happy with a 512GB “Lexar” brand (A2/U3 speed) card as “portable storage”, it recognised and read the files on the card, as well as dumping the usual android directories on it. I’m dropping back to a 256GB card for now, but might buy a faster 512GB one later if/when I run out of space (I like lots of music and photos on my phone)
The Lexar card came from AliExpress, the current 256GB “BlitzWolf” one from BangGood.