Keep in mind that this doesn't need to be a Raspberry Pi and can be any computer. With some networking magic (and potentially network namespaces) you can also run multiple dongles & proxies on the same machine. A bit of hardware engineering will allow you to decouple the SIM card from the modems themselves, so SIMs can be kept at a central location and assigned to modems at will, potentially automatically depending on remaining credit/carrier blocks/rate-limiting/etc.