How much will you fly in the king air job? If you do become asst CP, how will that affect your flying?

While your 800 TPIC is good experience, you really need multi TPIC. If you can get 1000-1500 fairly quick, you could always go back to get more 121 experience. If it's going to take 5 years to get it, I'm not sure it's worth it. Another downside of the KA job, it's a 200 so no type rating. Not that it's a big deal, but shows you can be trained to a higher standard.

If you get a chance, go to a job fair and ask about 135 prop captains getting hired. I'm sure there are some, but I suspect it is a very low percentage.