You are right - the MMS functionality is only supported by their web app so far: https://wiki.voip.ms/article/MMS . Is Google Voice an option for you or are you (understandably) hesitant to use their services for a critical/sensitive/family number? Based on my limited use, GV appears to support MMS including in group texts.
The latest trick is for spammers to send MMS and then see who responds. I am still getting texts from people on the thread to stop responding, although now it devolved into a political fight with two numbers constantly texting "Trump won". There is no way to remove myself either... I had to disable GV forwarding, but apparently this is enough of a problem that they are disabling forwarding anyway.
i ported my mobile number to them a week ago hoping to only ever need an LTE, 5G or WiFi data connection for cell service.
alas, i discovered that MMS (and therefore group SMS, too) dont't work through SIP protocol. that's a deal breaker for me, unfortunately. looks like i'll have to port it back out to AT&T, Verizon or T-Mobile :(