This is how most people think unfortunately. There is zero regard for anyone’s privacy, they just want something popular which works and feels intuitive. I’ve even had ignorant people insist that I must share private details about myself over a phone call rather than a Signal call. I tried explaining why Signal is better but they didn’t care, phone calls were more familiar to them and therefore the “safe” option.
I doubt your threat model is severe enough to warrant causing issues at work by refusing to join WhatsApp and trying to fragment the group chat over on Signal, assuming you could even get others on there to begin with which sounds like it wasn’t a success. My advise is to give in for now and if there’s any coworkers you can bring on Signal, you’re free to keep trying. If you don’t mind spending money on harm reduction, you can totally use a VoIP number or a burner number and use WhatsApp in a dedicated GrapheneOS profile while forcing a VPN or Tor to always be on in said profile.
Unfortunately RCS isn’t adopted across all messengers. I still have some conversations on SMS (maybe they’re on outdated phones?) and funnily enough, more privacy-conscious people using VoIP services like JMP.chat and MySudo can still only use SMS.
There also isn’t a clear advantage of RCS over WhatsApp since it’s currently unencrypted on most messengers (only E2EE on Google Messages?) and RCS metadata is most likely being tracked by governments and ISPs in addition to unencrypted message contents being wide open to interception. As it currently stands, you’re basically choosing between guaranteed E2EE with more metadata sharing versus occasional E2EE (unlikely in big group chats) and potentially less sharing of your metadata depending on how evil each participants’ government and ISP is.