Can you elaborate on Hangouts + Project Fi? I'm also on Fi, and want the iMessage type experience, but always seem to fall short of it. Is there something particular about the Hangouts and Fi combo?
Set Hangouts as your default text message app on your phone, text messages and voicemail transcriptions then get sent to Hangouts and you can see them on the computer as well as the phone. You can also send text messages from the computer.
I never used iMessage so I don't know how it compares, but this has always worked well for me.
I'm not sure if the option is in the Fi app or the Hangouts app, but I allowed Hangouts to manage my SMS on the phone. It behaves as my Google Voice number did before: SMS (as hoped) and phone calls (not so useful in practice) pop up in Gmail as well as on the phone.
This is what I do, and it's reasonable. Annoyingly, I sometimes miss calls as my computer will start ringing 10+ seconds before my phone, so by the time I grab my phone, the incoming call has already been sent to voicemail.
I never used iMessage so I don't know how it compares, but this has always worked well for me.