Apple Watch 3 has been my solution.
While I can’t draft out detailed messages using the scribble function, nor can I always use dictation, but I can at least see and ACK iMessages when I am out and about with only my Pixel 2. My Watch is still paired to my iPhone 7, which hasn’t had a sim in it since I switched my sim from that into the Pixel 2 as a primary phone for that one.
Not a cheap or easy solution, but it’s the best way for me to deal with both ecosystems at once. Odd thing is, the Apple Watch will not receive SMS or MMS from outside of iMessage, so there is one huge drawback there.