There’s an app currently in beta called Beeper that aggregates a bunch of chat apps into one and also lets Android users connect to iMessage using an Apple ID. Highly recommend getting on the wait-list for it, the app has been great and the team working on it are very responsive and transparent.
Unfortunately RCS support is still in active development so anyone using it may not want to connect their number yet (unless they’re cool with SMS’ing with other Androids).