I don’t believe you’ve really considered the implications of your concession that we know nothing about the physics of base reality. It means we’re not being simulated by carbon based life forms. Not by silicon based or any other element since chemistry all depends on quantum mechanics. Without quantisation of matter, the universe loses all scale. You could have universes within universes on the tip of your finger. Not that you’d have fingers.
The word “simulation “ literally means a “likeness”, but this world would in no way resemble base reality if base reality is not quantum, so by definition it is no longer a simulation. Perhaps some weird experiment in universe building by unimaginable beings but like I say, then we’re back in the realm of gods.
I think this belief is a cultural reflection of the contemporary preoccupation with computers and digital simulations, a fantasy in which we succeed in living inside those devices we stare into day after day. But if you think about it too much, it’s self contradictory.