Privacy help needed - my ex has my old phone on wifi only and seems to be able to imessage from it as me. Can he see my iMessages?

With my current iPhone or iPad, when I go to "Find My" I only see the four devices I'm actively and currently using: an iPhone, an iPad (with data), an Apple Watch (without data), and a pair of Beats earbuds.

Over the years I've had several other iPhones, iPads, etc. I always delete all the apps, change my iCloud password, etc. I let my little kids use the devices on wifi to watch YouTube, Netflix, etc. I also have an old iPhone that another family member stopped using.

I'm divorcing, it's contentious, and we separated last year. In the time leading up to our divorce, and especially since we have separated, my ex often seemed to know where I was, who I was talking to, and personal details of my life that I couldn't explain. I've checked my vehicle and home for tracking and listening devices and found nothing. I couldn't find anything on my phone to explain this (he is an Android user).

When he moved out, he took an old iPad and an old iPhone with him. Again these devices do not show up for me in my "find my" list because I've deleted them so they shouldn't be tied to my iCloud account.

The other day, my youngest daughter was at her dad's house, and she sent a text using one of my old phones he has there to another family member. That family member received the text and it looked like it was from me. How is this possible? It seems to somehow still be associated with my phone number? If that's the case, is it possible my ex has been receiving, taking copies of and/or viewing all of my iMessages without my knowledge?