If we didn't exist then indeed we'd know nothing. So your proposition is correct but since we exist, not very helpful.
True, but science explores the world external to the self, examines what it finds, and seeks to explain it. Aware of the problem you mention, science also seeks to maximize objectivity, a practice not notably followed by religions.
Incoherent? Neither you nor I act as though we think that's true. Instead we act as though a world exists external to ourselves, that our senses are capable of informing each of us about it, and that reason is a valid tool.
Again, you don't act as though the world external to you is imaginary. Instead you employ it for breathing, eating, socializing, posting on the net, and I dare say you wear clothes, carry an umbrella when it rains (indeed, check the forecast), avoid stepping in front of moving cars, are careful round electricity, get sunburnt if you get too much sun, see the doctor when you're ill or injured, and so on.
Yet again I point out that your conduct in posting here shows you don't believe that.
There are no absolute statements in physics, based as it is on empiricism and induction, and what is best opinion today may not be best opinion tomorrow; so what? You still need air, water, food, shelter, society from the world external to you, and you act accordingly.
And meanwhile science puts rovers on Mars and maps the brain and creates new materials and pursues understanding of the genome and improves medicines.
What has religion done for the world lately?