hesitantapplegal
I did this same switch, but prior to the switch spent months deep diving into how I would get it done. I went on the extreme side of all (privacy, security and anonymity) when I switched, for reasons.
Like you, I owned an iphone for more than a decade. I will say when I transferred the UI differences were very notable. If you didn't love it when you tried your ex's Samsung, you will probably have a similar experience until you get used to it. If you are determined to learn another interface you will be able to get comfortable with the switch. I was and I picked it up fast.
You can make your device as you see fit. The switch alone will give you incredible security and privacy benefits over IOS no matter what apps you decide to use. If you can't seem to break away from some of the brand name platforms like WhatsApp, that won't be a problem. You can use Google Play services and still be better off. You could also segment all Google Play services in an additional profile or private space, whatever you decide fits your needs best.
You won't have access to iMessage and FaceTime, but GOS has SMS and phone apps that work similarly. Or use platforms like Signal or WhatsApp. I do know that and iphone (using iMessage) messaging and android (SMS native to android) you will have issues sending pictures and videos. They don't send well across each other.
iMessage, FaceTime and Whatsapp are encrypted.
I recommend figuring out whether the contacts you have in iMessage can switch to another platform, because maybe they at least have Whatsapp. If they refuse to use another platform, then you will have to decide how to proceed.
Something to consider: since you said your threat level isn't that high, stock GOS SMS messaging to iMessage may not be a critical factor. You'll be on a more private device away from big tech.