Putting in new thermistat today. Know to be careful to not get coolant on timing belt. Where is a plug or other access to bleed off some coolant? How much to drain?

Passenger side, down under the back spark plug, looks like a brass fitting, slightly knurled to push on a 'catch' hose. loosen that a few turns' take off the cap from the Overflow and there you go.

It may be time to do anti freeze anyway.

OH, If you are changing the thermostat because your gauge isn't happy, It could be you compensation brd giving you a false reading. That's another story. If your gauge is telling you that you are too hot OR too cold it may not be the thermostat.

There are two block drains under, respectively, the number 1 and number 4 exhaust manifold runners, but why not just loosen the bottom hose at the water pump for enough time to drain the coolant below the theremostat level?

Just drain it from the lower radiaror hose, a lot easier to access than the block drains, and also easier to reconnect. This will be plenty below the thermostat level.