I’m not a doctor but I have had a few students who had knee operations before starting TKD, and they had no problems performing any techniques (however they were younger). Usually when doctors speak about “low impact,” it is related to compression impact, such as the feet impacting the ground during running or jumping. In TKD, the impact is usually from the joints stopping suddenly after a kick or punch (there is probably a medical term for this). However, in TKD you are constantly on your feet while twisting and turning the body in all directions, which is what medical experts recommend for people as they get older.
As you are experiencing, bruises are the most common injury in TKD. I’m not familiar with any bruise treatments other than the old reliable R.I.C.E. (Rest, Ice, Compression, and Elevation) that is recommended for sprains. There are some Chinese herb treatments but their actual effectiveness if questionable. If it is a deep bruise, you need to wear an arm pad over the spot to protect it from more strikes on the same spot until the bruise heals. Over a relatively short time, the arms will adapt to being struck and the number and severity of bruises will be lessened.