From what I think I understand, all information/data exchanges between Google RCS Servers and any Google Messages App (and/or between/to any "Web Browser session for Google Messages" ) is secure and not at risk. And of course, any direct Google Messages App to Google Messages App information/data exchanges are also secure and not at risk (as is also any iMessaging App to iMessaging App data /information exchanges).
I am not even sure I fully understand what all the not secure aspects are about the current situation. Perhaps only the part about how carriers might currently facilitate/forward RCS data/information exchanges based on telephone number as key, for between Google RCS Servers and Apple iMessaging Servers, and/or for between their own or other carrier RCS Servers, and from that perspective perhaps no different than how carrier facilitated SMS/MMS messaging has similarly for years not been secure.