I can agree with setting a bronze percentage which is more attainable by everyone, but I don't think there should be a prize purely for participating, or that the highest rewards should be available for everyone, since it just removes the challenge and incentive to try to improve. But in general i would say even as it is now at 10% it is attainable by anyone that is willing to learn the combo.
in my experience these time trials and their rewards have been really good. I only started playing with the release of gt7, and there was a learning curve to be able to gold the licenses, missions and circuit experiences. But I'm glad they were a challenge and pushed me to become a better driver, and the level that pushed me to allows me to get into gold for these time trials, and time trials are one of the few things that keep the game fresh and challenging.
As you say there is a large skill spread in this game, and somehow you have to cater to them all. And I think the way PD has done it by setting a percentage from the best time is extremely fair and smart. It means as many people as want to can get the reward, and it's not capped for say the top 3%. But yeah some people will need to spend a lot more effort to get into the reward zone than others. Personally I have spent anywhere from a couple hundred to several thousand (road Atlanta) kilometres. But most of the time all you need for gold is knowing the braking points, gearing and apex speeds, and a rough idea of the lines. You usually don't need track limit abuse etc unless you want to be in the very top.
Just like the circuit experience, obviously nordschleife is going to be hard since it is so long, but the time trial is there now as an incentive for anyone no matter how good to practice and learn and improve and see what they are capable of. Some won't make it into bronze, but I don't think it is fair to equate that to the system being designed to screw them over and not allow them to have fun. The fun of a time trial is specifically to learn how to drive faster to set the faster lap. And for some that just doesn't appeal, and that's ok since there is other content in that case.