The past and future may or may not be real; but the present demonstrably is not.
You haven't demonstrated what you claim to demonstrate.
Relativity of simultaneity means two people can look at the same events and see them differently.
But they are looking at the same events. It is not different events.
One present seen differently.
No. As many presents as there are points of view; many of them incompatible with each other.
If I walk around a building and see it differently that isn't evidence there is no present. To think so would be bizarre.
Lots of things are both true and bizarre.
There really is no such thing as 'the' present; every point of view has a different observation of the sequence in which things occur. Person A can remember something person B is experiencing, while for person C none of it has happened yet.
Yes, it is bizarre; and yes, it is observably true.
Human intuition is a poor guide to reality. We can either discard intuition, accept the bizarre, and be able to accurately predict how things work - sufficiently well to build tools such as GPS; or we can cling to intuition, discard the bizarre, and get things hopelessly wrong, such that our technologies simply don't work.
In a contest between your incredulity and Einstein's relativity, relativity wins, because it works, and your intuition doesn't.
The existence of the present is entirely a local phenomenon, and using the idea that it is universal to build an argument is doomed to fail before it even starts.
You are entitled to your own opinion; but you are not entitled to your own facts.