There are 2 concepts I can think of that are great candidates for grounding the discssion on the ontical vagueness of The Present.
Relativity of Simultaneity. For you, at any given time, there's a well-defined list of space-time events that are simultaneous relative to your reference frame - but relative to the reference frame of another person traveling with respect to you, some of the events you think are simultaneous are not simultaneous for them.
Block Universe. Many physicists think time is, not exactly an "illusion" per se, but also not necessarily a fundamental feature of the universe and how it operates. The way we experience "the flow of time", as if all that's true now is somehow more fundamentally real than things that will be true in 10 minutes, or things that were true 10 minutes ago - that's somewhat illusory in the Block Universe view. It might be that all moments in time are all always equally real.