The “official” response from Cricket support is using the @mms option you mentioned: https://community.cricketwireless.com/service-features-13/email-to-sms-gateway-13599
However, there have also been posts saying only about 40% to 60% of messages get through. You might also want to check the length of your messages. Maybe some titles or branch names are causing the standard SMS message to be too long which is why it needs to go over the MMS.
Over the past year, carriers have made MAJOR changes to their delivery of text messages in attempts to cut down on text SPAM (and to make more money for themselves). This includes needing to register your brand as well as the purpose you’re using to send the texts.
I believe that Vega LX Notices is intended to help with this but there are also other options you may want to investigate as I’m not sure of the status of that feature set.
These “helpers” usually come in the form of “central” email to SMS gateways. With those, your Polaris set up has one entry for all carriers like @sms-gateway.com , so staff and patrons only have ONE thing to select.
A couple of ones I’m aware of: