I am still not convinced on the long-term safety of ultrasound and dopplers due to the fact that they have been shown to damage mice fetuses in studies. I'm not saying to never use them -- I've already had one abdominal scan and one internal one due to the bleeding -- but to consider having fewer scans and less use of dopper. They also haven't been studied very much at the frequency that they are used today (they've been studied in the 70's and 80's, but not so much today even though they're used at a much higher frequency). I am definitely avoiding all 3D scans at least, since those are the worst.
This link worries me: https://chriskresser.com/natural-childbirth-iib-ultrasound-not-as-safe-as-commonly-thought It suggests that they can cause the baby's tissues to heat up (especially the brain), cavitation, and acoustic streaming.
Ultimately though, I would be careful about overusing home dopplers. Even if they don't cause any damage to you, wouldn't it be more stressful if you couldn't find a heartbeat? I totally understand wanting to have the comfort, though... Again, I totally see the use of them for the 20 week scan, if you suspect twins, if you're having bleeding, or any other high risk situation, just not for normal pregnancies when you don't suspect anything abnormal. Just my thoughts!