Rumor has it that Purism will be announcing something during the beginning of July (July 1 or July 4), so you might want to wait a week before you make your decision.
You also might want to read this thread to understand the difficulties with getting another cellular modem.
Since the Librem 5 is using a standard-sized 3042 M.2 card, it isn’t that hard to replace the cellular modem and add the driver if you are willing to use binary blobs.