I've been there in this situation myself.
I put down the buckets and picked up a python from Amazon. Or if you are handy, I got a submersible pond pump. I hooked a 1/2 in hose to the outflow connection and ran it to the nearest drain. It's going to drain the volume quickly. Then to refill, I reversed the hose to pull water back it too the tank. Iet a 5 gallon bucket full after adding dechlorinator and plugged it in and keep the bucket filling until tank was topped off.
If you would like I c post a picture of this set up.
As far as lowering my nitrate problem with similar tank size, I cleaned the filter more, added filter floss, and a bag of purigen. It worked well for me. You could also add some more bio media if possible. Or maybe a pothos plant with roots in the water in a small basket covering the roots with pebbles. There's a lot of different ways to control nitrates.