Maybe swiping up from the input field could also work, instead of the long press? Just a thought. Also not discoverable.
But yeah, this is amazing. End of the select text -> cut -> write whatever you need to write -> paste dance. I need this in my life.
This problem is potentially solved by making messengers more threaded, so you can reply to an individual message if needed, without losing whatever you’re typing in the main thread. See Slack, for instance.