As for your last statement about the round Earth being hit by rocks travelling at thousands of times the speed of light? Well, within a round Earth's frame of reference, there are no rocks travelling at that speed whatsoever.
And within a flat earth's frame of reference, no rocks would be "stationary", since that would be the equivilent of a velocity difference of thousands of times the speed of light, and that's no more possible in the FE universe than it is in the RE universe.
That's what you're choosing not to address when you say things like "A flat Earth covers more distance in a time period compared to the round Earth." Distance - like velocity - has to be measured from a starting point. You can claim D = VT, but velocity, too, needs a reference point. I choose that reference point to my my left foot; in that case the flat earth is moving 0 m/s, and will travel 0 m in the next billion years. How, then, can you claim it covers more ground than the round earth? I mean the earth isn't moving at all, so the only way a collision on the scale you're describing could happen is if a particle was accelerated towards my left foot at thousands of times the speed of light, and that's just silly.
How are you measuring the "speed" of the round earth, anyways? In relation to the sun? Well ok, but choosing the sun as your reference point is no less arbitrary than choosing my left foot. I assure you, relative to a lot of things, the sun is moving very very fast. So what, then, the center of the galaxy? You think that's not moving? Ok, so the center of the universe. What makes you think that's "stationary"?
That's a big, important concept we have to get out of the way; there are no absolute reference points. So when you say the earth is moving relatively slowly relative to the sun, I say that's nice but the flat earth isnt moving at all compared to my foot.
So no, the flat earth won't "cover more distance", and it won't be any more likely to encounter particles moving at the
ridiculous speeds that you claim.
The only argument you have for the increased likelihood of a rock hitting the flat earth at all involves surface area, and still that doesn't make it any more likely that the rock will be going faster than the speed of light relative to earth.