I hear you! That was by far the most difficult thing to "leave" behind when I started using Pixel phones and the one thing that kept me going back to iPhones in the first year of the Pixel line.
To be honest though once you start using Whatsapp you realise that it works just as well and is also cross platform so you get "free" messages with everyone not just your iOS mates.
The rest of the OS in my opinion is so much better that it easily makes up for the loss of iMessages. Buttery smooth operation, brilliant Google assistant, monthly security updates and more user options to personalise the app home screens etc.
The fact that the hardware is great too is just a bonus, class leading camera and brilliant wireless charger, all day battery life and a phone that is a joy to use for hundreds of dollars less. It's an absolute win win. I'm saying this and I've had every iPhone from the first one. Face ID was the first nail in the coffin for me as it is a real pain in my experience, the horribly buggy OS and being overtaken in terms of hardware innovation while hiking the price up was the final one...