@nogoogle I don't know why I didn't think of this before, but the whole reason that I left Telus was because they said that due to network changes, my device (then a Oneplus 5t) would no longer have functional MMS. https://www.telus.com/en/support/article/picture-and-group-messaging-changes
The linked article says "older decices", but other communication from Public Mobile said the only way to be sure was to use a phone from a fairly small approved list, which FP4 isn't on.
Maybe some kind friend who uses another network would let you briefly borrow their SIM for MMS testing, to see if changing networks would solve it.