You say you get nervous and lose time over-thinking the use of interfaces. And you seem to be seeking permission or an excuse to stop doing that.
I do not believe you need permission from anyone to stop doing something that does not feel helpful. So I suggest you stop immediately applying interfaces that do not present a clear benefit to you.
I am serious. You demonstrated you understand perfectly well what interfaces are about and what the benefits are supposed to be. While these are moot in your application, just ignore them and don't feel guilty. In time as the application grows the introduction of some interfaces will likely become appealing. That would be soon enough to introduce them and it won't be disruptive to what you have, the rework for the part that calls for interfaces should be smooth.
Now you will still have to deal with you OCD regarding inconsistency. Because you will have then have some interfaces for classes that feel good, but other classes do not implement any interfaces yet! Are you near a window? You may want to look outside for a while and wonder why the trees have leaves and the birds don't. And whether that is a problem or not.