The main thing I miss about having an iPhone is the ability to send full resolution pictures and videos over iMessage. In practice, SMS and MMS are seriously limited in the size of files they can send.
That won't do anything for Android-to-iPhone messages until Apple implements RCS. Almost all of my friends use iPhones.
As an aside: I actually don't have RCS on my Android phone because I run LineageOS without any Google apps, and apparently Google hasn't added an API for RCS, so there are no open source alternatives to the Google Messages app if you want support for it. (Please, correct me if I'm wrong!)
You don't really need an OS API for RCS like you need for SMS/MMS, as RCS works over IP network. If you want/need to use USIM based authentication, you need a system privilege, so that you can talk to the SIM card.
Someone has written an open source RCS prototype app, but it is only for carrier-provided RCS and I don't know if anyone has even attempted to use it with Google's servers:
