Another option is to use Instagram in Safari. You can bookmark the "instagram.com/direct/inbox" URL for quick access to your messages.

There's also an app called Beeper that consolidates all your chats from different apps into one feed. I have some privacy concerns about it, so I stopped using it ...

I'm also a European struggling with the idea of switching to a dumb phone. The main obstacle for me is WhatsApp, as everyone I know uses it. I do chat with people on Instagram as well, but I can easily do that on my computer.

One piece of advice I can give is to use your bank card for transactions ...