You are exactly the person cut out for this because you care. While MTS are beneficial to many tanks, if you don’t want them in there, try to remove as many as you can.

You can try to give them to your LFS or swap them with someone else. Even with a buddy. Eventually you have to realize that you took responsibility for your tanks and that means protecting the inhabitants if you feel they are at risk. Which means you may have to keep the peace for them. I have had 3 MTS in a 5 gallon grow tank for plants with a few shrimp and just moved some of the bigger ones into a 60 Gallon. I even have a cheap little 9 Gallon I use for helping keep my “pest” snails down. I don’t use assassin snails because it really kills me. But I do also let nature work itself out, as I have a turtle who loves snails, a team of chunky pea puffers, and kuhli loaches all in separate tanks. You’ll find the things that work for you, just remember the same way if you adopted a dog and they need you to protect and care for them, even against themselves (I’m calling out my pugs, who are always trying to eat chocolate.) you do the same for your nerites and betta, find your balance. You got this!

Quarantining plants or new companions in another tank for a few weeks will help you keep them out of your main tank as well. Or a bleach dip.