What is your preferred blood flow rate for a patient with ESKD who has an AVF but is only undergoing an ultrafiltration session?

I see no reason to limit the blood flow. Clearly, if you are just UF, the blood will "thicken" as you remove protein and cell-free water component of the blood, and the higher the blood flow, the lower the filtration fraction (FF) and the less "thick" the blood will get. If your UF says 3 liters ove...

It really doesn't matter very much. I don't believe higher blood flows pose less risk of clotting the dialyzer. Normally, I go with around 200-250 ml/min. If there are problems with the flow or clotting, then I would try lower BF first to see if it works better (mainly trial and error).