Your first idea is the best, as caulking, the holes with boards is the safest option. My fence had just as many gaps, and I didn't like it. I thought about building a new fence, but it is too expensive. After analyzing all the options, I decided to buy special garden screens here,
https://www.luxuryscreens.co.u..., and install them so they would close the gaps. My idea worked perfectly, and now people outside can not see what is happening in my yard.