I am a relative noob (less than a year) and like you, have been doing intramuscular via 21G needles. But because of possible muscle damage over time I've ordered 25G 5/8" needles and will start doing subcutaneous injections instead.
I'm doing this for 2 reasons. One like I mentioned, as many have described on the forum, IM injections can cause scarring. And two, recently I've had hard to explain erratic levels and symptoms. So my thought is, if you are shooting in the same spots (even though you are rotating) those spots can develop scarring or hardening, and (this part is only theory on my part) this possibly is affecting absorption and levels.
So when my new needles get here I will be joining the SQ club.