I've previously gone through review for IG basic + messaging permissions, it took several months of constant submissions to get approved. I'm not sure if their process has changed (got approval ~1 year ago), but we quickly realized Meta's entire review team has been outsourced to external contractors that just don't care. We'd get random rejections relating to functionality and check access logs to discover they hadn't even opened the app.

What ended up working for us was just keeping consistent & simple responses to the usage questions, and whenever we received a rejection with a valid concern, we'd address it at the top of our existing response with something like "In response to the latest rejection: ...". Unfortunately it seems unless you get a competent reviewer, you're out of luck, so just keep submitting as soon as rejection comes in and eventually you'll get through.