I found the shinobi-goma (stealth bridge?) to be a not very good solution for Tsugaru practice. The action is much higher than a typical Tsugaru bridge, which throws off your bachi hand, especially for sukui. On top of that, it does nothing to silence the thwacking of the bachi on the skin so that all that’s left is a loud, furious hammering sound. I think it was originally made for other shamisen styles – it might work fine for jiuta players since they use a relatively high bridge and don’t hit the skin a lot.
Some Tsugaru players put a mouse pad over the skin. I’ve tried it a bit myself, and it’s not a bad solution some of the time. The other thing you can do when you need to be quiet is work on your quiet playing, with your right hand pinky on the koma.
But you’ll want to put in a lot of ‘normal’ practice too, so sometimes you just have to make noise. A lot of musicians in Japan seem to practice outside, away from people’s homes. When I lived in Kyoto I used to go play at the Kamo River several nights a week (after I’d already driven my neighbors completely crazy). If there is a river or a train yard or a factory district anywhere near you, that might be the place to try.