I was exactly in your situation about a month ago (CYA 150+, also high CH). So I rented a sump pump from a pool store and drained it until the entire shallow end was dry - I estimate 2/3 of the water out.

It took about 12 hours and my pool is a little larger than yours (20K gallons). Took about the same time to refill using garden hoses.

At the time, I was very concerned about this, but turned out just fine. Water has never been better; it's great to have everything back in balance!

Something to check: here in No. Cal. you can't drain the pool water down the street and into the storm drain; but, you can drain it to the sewer. Look for a sewer clean out and direct the flow there.