To fly part 135 as pilot in command you have to have 1200 total time and 500 cross country. Note, it's just point to point. NOT the 50nm required for ratings.

Some places offer SIC programs but be skeptical of most of them. Many require you to pay for the time and this is not looked upon kindly. Many are also not legit as the planes they are flying only require one pilot and because of that you only get to log empty legs. I think Airnet is one of the few that pays you.

Instructing is a great way to gain experience. It will make you a better pilot if you're wise about it. It's not about doing touch and gos and ground ref all the time.