It's a built in tank (basically space inside the keel) so removing it is not an option
I can get inside through a few smaller holes (a 1" diameter access plug, or if I remove the sender unit - through there). These things sit on top of an access plate, which if removed would get me access to the tank - but from past experience taking similar plate off of the water tank, I don't want to do that. It is bolted to the fiberglass surface with bolts glassed in - when unfastening the nuts, many of these bolts will start spinning and then I have to dremel them out, and re-epoxy - very long story and in this case complicated by access, since tank is under the engine and clearance is marginal).
I will look for cheaper pump if I can find one (I didn't have much success with drill pumps in the past) and try to get to the bottom of the tank through available ports - if this gets even only 3/4 of the sludge out of it, that would be great.