I have a seedling about as tall as yours I want to graft also. BUT I want to keep part of the seedling just to see how it turns out. It started as a seed from Zutano. I have a Kona Sharwill Type B tree also. It needs a type A for pollination. I am going to buy a Jan Boyce Type A tree from Atkins Nursery in Fallbrook, Calif, and use it to graft to my seedling. My seedling has two trunks. I'm like you. No clue. There are several informative videos on You Tube about grafting and top working avocados. The two I have chosen are both gourmet avocados. Very buttery, flavorful with small seeds and lots of meat.

With that size of tree...I would recommend a cleft graft. Google that. I have both Sharwil and JB. It is important to grow varieties that taste good and you will enjoy. I have 20 trees with 16 varieties. One of my favorites is Pinkerton. Tree shape/size might be a consideration, as well.

I looked at two cleft graft videos from you tube. I loved the demo, but he shows grafting in a greenhouse. He says to keep it in shade till the graft is really growing correctly. What can I do? Our seedling is in ground and taller than me! This is the one I'd like to copy: https://www.youtube.com/watch?v=fIl6IOt7z1g We will be field grafting.