BMI works quite well for typical people.

Either you do so much more exercise than everyone around you that it’s not a good fit for you, or you should take it as a warning sign.

If you think it’s muscle and not fat, there’s another test that you might like to try instead which is the waist to hip ratio. www.healthline.com/health/waist-to-hip-ratio

But if you want an honest appraisal of your fitness, just do a fun run. The shortish runs round a park with a bunch of normal people. Either you can keep up without killing yourself and everything is fine, or you’re not as fit as you think.