Everything ahead of us is of course the future. If I wish to travel to northern Michigan for the weekend, that trip would be considered the future until I arrived at my destination then it would be the present.
In applying the same principle to a universe that is expanding due to a gigantic explosion that occurred at point A (the center of the universe) and all the debris flying outwards, assuming I have enough time to do so, I decide to plan a trip to the outermost piece of debris in the universe. While it will certainly take a long time to get there, lets say I do.
Are you saying that once I reach that outermost piece of rock which will then become my present, I can't go any further?
If I am still alive (which of course I am in this narrative) my venture beyond that outermost rock would then comprise my "present" on a journey into the future, just as every day life is.
So, either the universe is forever and ever and ever, OR there are boundary limits that have only been explained theoretically via mathematical equations.....
This is incorrect. In the analogy, Michigan is on the same geodesic surface that we are on. Michigan does not exist in some future time when you arrive there. It is not unreachable because Entropy bars us from travelling there in the past. It exists already. Is exists in the now. The fact that it will still exist when you get there is a consequence of the fact that in travelling to reach it, you can move along a geodesic curve of spacetime to reach it, because it is travelling into the future along with you. If it did not, it would become unreachable.
Don't switch back and forth between the 2D/1T spacetime of the model and a 3D Earth. That mixes the model and invalidates the analogy.
We cannot visualize a four dimensional universe. But we can visualize a 2D/1T slice of it. In that slice, the radial axis is time. When the galaxies move through their spatial dimensions they are restricted to the surface of the sphere. When the sphere expands, all of space, and all of time are expanding along with them. An astronaut moving between galaxies cannot "jump off the balloon" and into the future. All he can do is stay on the surface of the balloon to travel from one spot to another. As he does so, the balloon, the galaxy he has left, and the galaxy that he is travelling to have all expanded outward in spacetime together.