I have always avoided multiple monitor setups for this reason, however, I don't think the size of my monitor affects things as much as my usage of it.

Have you tried using a big monitor but just keeping windows as full-screen? I find this to be much less distracting. And of course, phone face down and notifications turned off.

Not my app, but I'll plug Dayflow here (YC backed). It's really great for measuring what you're working on, and they have local only mode for full privacy.