How to make my meatballs more solid
a few tips here


I noticed that the beef was "80% lean", try to go with little bit fatter beef like "60% lean". This sounds strange, but I would try to beat up (or stir) the minced beef before mixing with other ingredients. When you beat or stir the beef, you will notice the fat will create a gel texture that helps to stick the beef better. 
I would also separate the egg yolks and egg white. I would put the egg yolk in 1st with all the ingredients and put the egg white last. Egg White on its own will also help you to stick the minced beef better.
Lastly, if it still doesn't work, try to lightly apply flour on each meat balls before cooking. The flour will help sealing the juice inside the meat balls, which the meat balls will stick together better