This app was mentioned in 6 comments, with an average of 2.00 upvotes
I’d recommend Unseen, which lets you read messages in apps like Messenger, WhatsApp, Instagram and the like without letting the other person know. I mostly use it to get a bit of time to think through replies to friends’ messages which, given that I’m a therapist to most of them, is a massive godsend. The biggest issue I have is the ads, but they can be removed for a quid.
I started using notify but still have the problem. So I read in a telegram group that someone use a different app like Unseen to get the notifications so that way it doesn't say "x notifications of x chats". I use notify putting the WhatsApp icon to Unseen and that is what I do.
Link to unseen(play store): https://play.google.com/store/apps/details?id=com.tda.unseen
Try Unseen Unseen - No last seen or read
It only supports a handful of apps but messenger is one of them