your almost at 1000 tt so the ATP is only 500 hours away (like you already didn't know that) as long as you slide that 500 x-country in there. I'd say you'll be there w/in 2 years so study up and take it.
Sounds like you have the whole summer, so take a little brake then get the red Gleim book and dig in. I recommend that you sked the test a month ahead this will give you reason to study. Set a sked like one chapter every 2 days and before you know it you'll have all the questions memorized before you finish reading the question. Also just do the 135 test, it doesn't matter after you get the rating and most likely if you get a 121 interview before you get the rating they won't care which one you took anyway, they just care that you can show them you can study aviation stuff and do well on a test after.
Don't be intimadated by that thick book, most of it is diagrams and other stuff that you will never look at. A bulk of the questions are right from the comm written and the rest (of the non calculation ?'s) are common flying/aerodyanmic knowledge. As for the long weight and balance and x-country questions the 135 test has like 3 aircraft (CE-208, G1159 and BE1900 maybe one other I forgot) with about 10 questions on x-country and w&b on each. You don't need to do the calculations as each is easy to memorize. For example the caravan goes from LAS to LAX. Its CG is this and the time is that just make a story with the answers so when you get a question on the test you'll know the answer. Do this for all 3 or so aircraft, it will save time and also the answers are so close that if you do the calc and round wrong you might picke 1.35 for time enroute instead of 1.4 which was the correct example.
O well gotta go and start a 4 day, hope this helps
PS - just study the book, don't waste $$$ on a course that gives you the same thing on a computer.