OOP with specialized classes as subclasses

Hello, I am making a Unity project that is quite simple, but it will get reused a lot of times with small adjustments.

It is not a traditional game and therefore difficult to explain without breaking an NDA. I am trying to restructure my code into more reusable, better code and I was wondering to what extent subclasses are useful.

Example 1: I have a script X that takes in a string message and I want it to call X functions in the same script. But in a similar project/scene I want the same script A to do X+ 3 more functions. Currently I am using bools to enable and disable behaviour, but I was wondering if making subclasses is better, both performance-wise and readability-wise, as it is unecessary to check the bool every frame, when it will constant during the whole scene. Would it be good practice to make a subscript (subclass) B that inherits from script A and just overrides the function? I am not sure what is the best practice to do this, even with subclassing. Do I override/overload the function or do I need to completely rewrite the function?

Example 2: I have a rotating object. In some scenes/projects I want it to snap into place, sometimes I want it to interpolate. I have now made a bool that enables/disables interpolating, would it be better practice to make a subclass that is called script XInterpolation and put the extra behaviour there? If so, how do I just "add" to a function. AKA, I want a function from the base class to run, but also additional behaviour. Is this through overriding?

It is not behaviour that will change during runtime. It will be build-specific.

Also, I have read that Start() is just called in the lowest-level subclass, but upon testing this with a simple script I got that both Start() functions were called. Is this an "unstable" behaviour? Should I assume that only the lowest-level Start() will get called? This is not with virtual void Start(), but void Start() in both baseclass and subclass.