What are the privacy implications of adopting RCS?
RCS is not encrypted. At least the non-Google version is. I think Apple is working on that with the GSMA or whatever they’re called.
So, Signal > iMessage > Telegram > WhatsApp > SMS
Is there any other apps that have RCS support on Android other than Google’s own messaging app?
I think Samsung. But then you need a Samsung, which is its own problem.
Otherwise no, but there may be more soon since it’s being adopted by Apple.
The only company that can acquire Signal is another non-profit.