Coming out to a straight cis male partner

Long time reader, first time caller, as they say! I am hoping for a spot of advice about coming out as non-binary transmasc to a cis male partner. My squeeze and I have been together for 22 years and have two kids under ten.

I'm not quite sure when to count my untethering from the binary from: it’s been at least 4 years, a bit before the conception of our second kiddo--although who am I kidding? Between regular weightlifting, dressing in what feels to me like neutral-ish way, and never having a period (tks IUD!), I was just treading water and trying not to address my feelings for years. I am now almost two years sober, which sure gave me a lot more time to think and nowhere to hide out from myself.

My squeeze is pretty supportive in all things queer: e.g. we both see other people, which has given me a lot of room for fulfilling queer relationships; he goes to LGBT+ rights rallies; is great on pronouns with our friends. I've been binding consistently for a year now, so it's not like he has no idea that anything is up. We normally talk about everything, but even though I am straining at the leash to start the steps towards top surgery and perhaps testosterone and to change my pronouns from she/her to they/them, I am nervous to bring it up to him. I'd love to ask for people's experience in similar situations and for any advice you might have about getting off on the right foot.