Ah wow, I didn't realize that contextMenu was implicitly utilizing LongPressGesture! Now I can simplify things.
I applied
.contentShape(.contextMenuPreview, RoundedRectangle(cornerRadius: 25.0))
to my tile and that worked perfectly.
Thanks a ton. You all are doing awesome work with the frameworks behind the scenes; finally glad to be interacting with an extremely solid UI framework for spatial computing!