Keep your iPhone 7. Get a new battery when the current one isn’t serviceable. The iPhone 6s was my favorite (128GB Space Gray). Unfortunately, I left it on an airplane.
I had replaced the battery in it twice and got the screen replaced once. I still love that iPhone the most. At 4.7” it just fit in the hand nicely. It was fast. It was a world phone. And it even had a headphone jack.
Now, I have the Xs Max. I cannot stand the notch. I loath looking at my iPhone. I try to not look up. I try everything to unsee it or forget but two years in I still see it all the time.
I would hang onto that iPhone 7 if I were you. Love it and appreciate all the good tech until something happens to force an upgrade. Next year, hopefully, the notch problem will be solved. The iPhones will get bigger and smaller at opposite ends. The screens will all get the same tech. Hopefully, we will get USB C. The USB C would be nice to backup files, but I am not as concerned as some people with having a one-cable solution. The AirPods and PowerBeats Pro both need Lightning.
So wait for next year. This year was rather unexciting with exception to the battery life. That is the only feature I would have loved on my iPhone Max. It dies before the end of the day, sometimes even with the battery case.