I don’t know why you insist on holding onto a 30+ year old protocol. It’s not perfect and at times it can be downright unreliable. Once it’s left your phone you have no idea if it was successfully delivered or not, there’s no acknowledgement no retrys no retransmits. It just shoots it off and hopes for the best.
Group chats are laughably broken even among all SMS recipients (It was never intended for it anyway) and frankly the bandwidth required for text regardless of if it’s over SMS or RCS is inconsequential, who cares if RCS messages need a bit more bandwidth to send text. The difference is negligible.