Have you tried Bandwidth
You might have done your research already but to quote a paragraph on some impediments, from https://www.nowsms.com/faq/how-mms-works
The problem with the Direct MMS Delivery approach is that the MMS client on every mobile phone is pre-configured with settings for how the phone sends and receives MMS messages. To send or receive an MMS message, the phone makes a GPRS connection (to a GPRS APN). It then usually connects to the MMSC for sending/receiving messages through a WAP proxy/gateway. The pre-configured MMS settings on many mobile operator networks are setup to connect to a special MMS-only GPRS APN which connects to an MMS-only WAP gateway … and this GPRS APN/WAP gateway is configured only to allow connections to the operator MMSC. If the recipient mobile phone is subscribed to an operator that has this type of setup, and you attempt direct MMS delivery, you can send the MMS notification to the phone over SMS, but the phone cannot retrieve the MMS message from your MMSC server because the GPRS APN/WAP Gateway does not allow it.
MMS is very tricky even Google Voice struggled with this for years as has lots of telecommunication providers. What usually happens is the recipient ends up with a link to access the message MMS in a browser.
There are alternative cheaper and quite reliable ways of sending multimedia messages but your motivations might vary. Make sure you do a cost benefit analysis as well.