Cheapest way to get IMessage?

See title. You can also respond to the context if you want.

Context: I currently have an iPhone 11. It is starting to slow down and the battery is starting to be an issue, so I’m considering replacing.

I am currently on ATT. I think they currently have $1000 trade in promos for Galaxy S24 and pixel 9 but only $350 trade in promo for iPhone 16.

Really my only things making me reluctant to leave apple is iMessage. My little sister(15yo) doesn’t have a phone, but does have imessage. Also, my girlfriend and I sometimes send gifs on imessage. My gf and I can adjust to normal SMS, but but my sister not having a phone is a bit annoying. We don’t talk much, but I think it is important that I am available. So it might make sense to find other ways to access iMessage.

So my options are

1) stay on ATT, get a Pixel or galaxy, find a cheap way to access imessage.

2) wait for iPhone 16e, consider switching to a cheaper phone carrier.

3) stay on ATT, get Iphone.

4) switch carrier, get cheaper phone, maybe still find cheap imessage access device.

1 or 2 seem to be the front runners.