Hi and welcome! Diego is a munch!
About the harness. If you are trying to correct pulling, the harness is a definite NO-GO! Harnesses allow dogs to pull BETTER! They can get their whole chest into it, which is much stronger than their necks. It actually can encourage pulling.
You will just have to do some basic leash training. Don't walk unless Diego isn't pulling. It can be maddening. I had a foster, Dudley, that was the WORST puller. I would walk 2 steps, he'd start pulling, we'd stop. One step, stop. A walk around the block could take HOURS. hahha! But, they have to know that unless they're being nice, the walk will not happen.
You may want to try a Halti or Gentle Leader to help with the training. There's a good thread on it here by our friend Jeanne.
Nice to meet you!