I'm typically agnostic when it comes to picking languages. Whatever gets the job done, but, in this case, to figure out what's involved with what you want to do, it's going to be much more than just picking programming languages for your client and backend interfaces.
So, I'm recommending you use Flutter+Firebase as a way to figure out what you need to get done. It supports both iOS and Android out of the box, introduces you to a bunch of components you will need to think about in the future (whether you stick with Firebase or not), and the key here, is that there are tons of articles, youtube videos, and other courses you can take to pick it up pretty quickly. You could even clone an Instagram knockoff from github. Guess what else, it's all free. You might not need to worry as much about the backend since there are client libraries for Flutter/Dart for Firebase.
Some might have different opinions, and like I said, I'm usually agnostic, but in this case, you have a lot to consider. Where are you going to store the data? Are people going to need to login? Will there but customized settings the will save even if I close the app? Yeah, that's just a few questions.
Those are just a few. Lots to consider, so if you want to get something in your hand as soon as possible, try a search for flutter + firebase + chat + Instagram or something like that and have a look.