What happens to a circle with a large diameter, as we increase its diameter towards infinity? When does the circle actually cease to exist?
I'm going to address this in more detail.
You are talking about what is known as a limiting process. Even when the limit exists (and it may not), the limit may not have the same properties as the individuals.
In the case of a circle of increasing diameter, the specifics depend on what happens to the center of the circle.
If the center stays the same and the radius increases, there simply is no limiting curve (this is not at all uncommon). There is simply no curve that the circles are getting closer and closer to.
If the center moves in such a way that some point is on all of the circles, the limiting curve is a straight line. This is a case where the it does not have the properties of the individuals. Each individual is a circle, but the limit is not.
Again, it is common for limits to not have the same properties of the individuals in the sequence defining the limit. In fact, this is a common construction in math for unusual behavior.
For those situations where the limit *does* have the same property as the individuals, we say that the property is 'continuous' with respect to that limit.
In general, when taking limits, it is necessary to be very careful. Some limits don't exist at all (there is nothing that things are getting close to). Others, the limit has different properties than the terms (maybe even different cardinality, volume, etc), and sometimes the property is continuous and the limits do have the same property.
Yes, it is possible to take a limit of *some* sequences of circles and get a line. but that doesn't make the line a circle.
If you want to do projective geometry, we *can* have 'lines at infinity'. But that is NOT the same context as Euclidean geometry, which does not have such lines.
There are more possibilities than you have imagined in your philosophies.