Thanks for all the info in the replies so far.
How I did the 200ft course was I went to a church and used a straight section of parking lines. I eyeballed the ends of the lines from the ground to make sure they were straight down the row. I started at one end and marked the center of the line at the end as my 0 point. Using a 100ft reel tape I marked the 100ft position. Then I moved the tape and started at the 100ft position then stretched it against the lines to the 200ft point and marked it.
The wheel was set up with 2 bands of flagging tape 180deg apart.
To roll the course I started with one of the flagging tape lines on the wheel at the 0 point. Started to ride as straight as I could at about 5mph and counted the flagging tape sound going around the wheel (only one of the tapes was making the noise I counted). When I got close to the end I was a bit past the 1/2 way tape hence the .6. If the 200ft point was right on the 1/2 way tape it would have been .5 fo a 1/2 revolution.
So there is some inaccuracy in the guesstimate of the .6 value, for sure.
There is also likely to be a bit of inaccuracy in the placement of the markings for the 0, 100, and 200ft points that accumulates.
So back to the course length question - by using a 100ft increment tape measure from the get-go there is going to be some cumulative error in every 100ft increment. So by measuring off 1000ft instead there is the chance of 9x the error whereas at 200ft there is only 1x (the error between the 100ft increments).
I suppose a surveyors wheel might be another way to measure greater than 100ft.
In the tape-around-the-tire method - that is not factoring in tire pressure while loaded and riding. That's why I won't use that method. I don't have a way to calibrate for tire pressure. If there was a way to know the change due to fluctuations in air pressure from the start (like calculating the velocity factor in coaxial cable as it affects attenuation of RF through the cable - the better the dilectric and less resistance the higher the velocity factor - less than 1, the worse the dilectric and more resistance the lower the velocity factor) then it would make more sense. However, the only way to get that error per tire pressure setting is to do an accurate rolling measurement with each pressure level - which is what I am already doing with my riding pressure/load from the start.
As to the GPS distance calculations rounding corners - I can see that. It totally makes sense.
As to the GPS distance not factoring in the "angular" distance of ascending and descending slopes (as opposed to the horizontal plane distance on level ground) - that I can see as well.
So the general theme here is the wheel rotation counts from a sensor are the most accurate method of calculating distance. So the question of "the most accurate" distance achievable comes down to the accuracy of the calibration of the wheel counts. And that comes back to all the sources of error I've outlined in the measured course I've been doing vs the tape-around-the-tire not taking in to account tire pressure and loading while riding.
I suppose thinking through all of the above - the 200ft course, for me, is "good enough" and it is more trust-worthy than any distance not obtained with a sensor.