DDCCing to say that I'd stay on it. Just my opinion. If your levels are decreasing, it could be caused by any number of things, from draws at different times of the day, to stress level, etc... but, it could be that your body just doesnt naturally produce enough to sustain an otherwise healthy and "meant to be" pregnancy. (though, your levels do seem good. really good. But a decline made me beyond nervous in my own experience.)
I have low progesterone naturally, and I also had decreasing levels in this pregnancy. (went from 38.1 at 13DPO to 9 at 21 DPO) And in a pregnancy that's otherwise healthy, progesterone supplementation HAS been shown to maintain viability. I'm not saying that because they do it so often, it "must be safe" but many mamas take progesterone supplementation to maintain a pregnancy in the beginning of pregnancy. and, IMO, the incidence of ill effects are the exception, not the norm. You can find bad side effects of anything if you look hard enough, even if it's generally accepted as a good, safe practice.
My Dr wanted me to stay on until 12 weeks with my 2nd pregnancy, I stayed on until 13 since that's when my script ran out.
Also, going on and then stopping could
cause an drop in progesterone before your placenta is reliably producing it on it's own. And a sudden drop in progesterone is the hormonal sign to the body to start menses. Just a thought to throw out there. Obviously, your baby, your body, your pregnancy, but I just wanted to give you more information from a pro-progesterone supp perspective!
If you were doing any harm to your baby with the supplements, I'm fairly certain your MW wouldnt prescribe.
*this is not medical advice
Good luck with the rest of your pregnancy, and many healthy and happy