Cannot group text or send attachments to Android

I've had a number of issues that have not been resolved in the 9 months that I've had my iPhone 14 Pro:
1) Cannot send group texts to Android users
2) Cannot send attachments greater than 400KB via iMessage
3) Cannot send attachments at all via MMS (i.e. to Android users)

Originally I could not receive texts from android, but I gave up my old phone number and went with a brand new number, so I was able to resolve that issue (essentially fixed it myself after hours and hours of dealing with Apple/xfinity).  I just spent an entire day on the line with xfinity tech support, and though they try to be helpful, in the end they just say "we checked all your settings, it must be an Apple issue, call them."  That makes zero sense, since Apple deals with iMessage only - and my problems are with SMS/MMS, which is the carrier.  I believe the attachment limit is also set by the carrier, and 400KB seems a little low - "we'll sell you a new iPhone 14 that takes high-resolution photos (that can't be reduced except by running them through a third-party app) but you can't send them to anybody."