It's not likely that you'll be able to block in-video ads with Pi-hole. For that, you'll need something like uBlock origin for desktop, or
vanced for android (Now defunct), or pay for premium
This thread has been closed as there is not much more discussion to be had around the topic.
User's have had good success here:
Blocking ads on YouTube is hit or miss with Pi-hole. The reason for this is because ads are typically served from the same domain as the video is.
Many users see unique domains in the logs such as
r4---sn-vgqs7nez.googlevideo.com and have had some success in blocking these, but there are so many and it's different for different people, so it's not foolproof.
Also, within this thread, you'll find lots of scripts and regexs, which may or may not work for you. Feel free to explore and report your findings on this thread.
Bottom line: blocking YouTube ads is a moving target, and you'll probably have the best luck crafting a complex regex to add to your blocklist.