You're not going to go from 135 cargo to mainline. The most you'll be able to jump to would be low cost carriers. Spirit, Allegiant, Frontier. You're already instructing, keep building the time. You'll get to 1500. If it's not going quick enough for you, there are myriad academies across the country that are doing 80-100 hours consistently, maybe even more. As you've already stated, you're not going to get that much time at a 135, and you'll have to work up from SIC (which isn't worth much when it comes down to it) to PIC. You'll go to a regional quicker as a CFI, and honestly you'd even go from CFI to Regional to Low Cost Carrier before you'd go from 135 cargo to Low Cost Carrier.