Training better leash behavior

Hi all,

I have a 5 month old pug/Shih Tzu mix. The outside is still super new and exciting, and although he’s respectful and follows commands inside I’m really struggling to get his attention outside. That, and he pulls like no dog I’ve ever had. I got paranoid about flat collars because my family dog had early tracheal collapse (my mom never trained her at anything, so she just pulled constantly), so I’ve only been using a harness. Clearly it isn’t working.

Any tips/video help is greatly appreciated.