I only drain maybe 3-4 gallons from my 40 gal water heater occasionally in a bucket just to get any sediment. Don't understand why you'd want to drain the whole 70 gal. I just use the spigot at the bottom and let gravity do the work, no pump needed. And every 2 years I'll check the anode and replace if needed. It's a bear to get the original out. Anodes are designed to be sacrificial - most people ignore them but it will add life to your water heater if you replace as needed.

I've always drained the entire tank. The amount of sediment is minuscule and I would not hesitate to drain the tank to your sump and then pump "overboard." After flushing the tank, I shut the bottom drain and add about 18 inches of cold water and flush that water out also - I suspect that will help remove any remaining loose sediment.

If heater is in basement, run drain hose upstairs and then outside down to tank level or better below and use siphon.

If you are going to drain it into something and then pump the water out, I recommend this Ryobi pump from HD. It moves a lot of water and will handle both clean and dirty water pumping. https://www.homedepot.com/p/RYOBI-18-Volt-ONE-Hybrid-Transfer-Pump-P750/300258095

I used it several times after rain during my landscape work and while I had a big hole in my yard before they poured footings. It works very well. I also like the option of using a battery (separate purchase) instead of A/C power, since wading around in water with an A/C device just makes me uncomfortable.

Bruce