How heavy should I go on those deficits?
Hard to say, I mean I only ever really did low rep deads, kind of miffed I never tried the 500×20 thing. I think tommywishbone did that

But anyway, if for strength I would do them in place of reg deads for a period of time, do the same rep ranges you've found beneficial for reg deads. If someone tends to start hitching when maxing IMO deficit deads take care of that very well, it helps improve form to "comp standards" almost automatically. You can't really hitch a deficit, at least I never saw it. Personally I did only one max set of any variation of deads really, but some like a bit more volume, some even do 5x5 LOL. Some powerlifters thought they could only do circa max deads every other week. My buddy who even managed to get gold in deads in his category at IPF worlds once did 4 extremely hard, almost idiotically so, deadlift sessions a week sometimes

For improving the dead and if wanting to try deficits for a while I might suggest one hard set of deficits, maybe 2-4 reps, one or two hard sets of good mornings, some nasty leg curling motion like glute-ham raise. Then some heavy calf moves. If there's still energy left maybe some chest supported rows or other upper back exercise. Once a week

I've lived for deads my whole life so I have a lot to say about the subject

I've had some nasty upper back spine pain the last few years but to my amazement imaging showed no herniations at all

Told doc I've lifted heavy for a long time but they have no idea... like 300kg+ (660) loads weekly for 25 years straight, but unless you lift yourself it's just meaningless numbers LOL.
Love rack pulls.
Nice. Makes you feel alive

When I was 15 years old and starting hard deads I went insane from the very beginning. People asked me if I'd been in a fight the days after, broke so many blood vessels in my face it looked like black eyes.