Hey, I solved the problem after a bit of thinking and tinkering!
It turns out that it had to do with the internal resolution, which kinda makes sense since playing at 640x480 was fine. First I doubled it to 7680 x 4320, and to my surprise it worked, except that my laptop wasn't powerful enough to run the game smoothly like that.
So, I tried a variety of 16:9 resolutions above and below the default. Some still had the artifacts, some made different artifacts (on the main menu), some caused the field textures to have big horizontal lines, most distorted my HD field textures to look a bit jagged rather than smooth, but
finally I found that 2560 x 1440 is a happy middle ground that (so far) has no artifacts or distortions or slowdown.
Now I can play Reunion R05c with HD textures