Great App and serves a useful need, I used to use inspect to test mobile devices no longer need to with this.

Some recommendations:
-For privacy focused people who dont want to use browser extension, consider a desktop or web app so we can access and test sites without using a browser extension which normally shares data with that browser like Chrome for example which privacy focused people hate.
-SEO score would optimize and further by adding site speed and adding page speed scores for mobile and desktop can get free speed score using google page speed api