I used my bicycle to visit every library within about 10 miles of our home. My interest was mostly in technical books. I read so much that the standardized tests they gave us back then indicated that I was reading at high school level while I was still in elementary school.
One of my grandsons has many characteristics, both positive and negative that are similar to my own. He is not all that sociable, so when he comes over for Christmas or his birthday I often hand him a kit to put together. He almost always has them put together before we start eating. When he was a toddler, he was completely fascinated by automatic doors when we took him to a restaurant. His dad was annoyed with this, but I correctly assumed that his curiosity at such an early age was an indication of intelligence.
His older brother is now an adult but always had more interest in mechanical processes. I had him weld his name on a steel plate with a MIG welder when he was eight. I just told him it was like drawing with a crayon only you had to keep the flame slightly above the surface of the metal. Our daughter was of course horrified but was pretty amazed by his aptitude.
I wasn't allowed a bicycle until age 10. I just walked. I had a similar experience in 2nd grade after all the copious reading. I wasn't informed of the purpose of the testing and after almost 4 hours of what I interpreted as "badgering", I shut it down with the spoken word "chilly". I very well knew that chilly, Chile, chile and chili were 4 alternative words with different meaning, but I was just tired of the badgering. I was reading at 7th grade level. Perhaps if they had explained the intent, I would have gone beyond that. No big deal.
I think you made some good choices for your grandsons. I'll be entirely missing the grandparent experience. Life just worked out that way.