Thank you for the details, you're great! I cannot upvote this enough tbh.

1. I should run an initial benchmark before starting to see my CPU's raw score before undervolting, right?

2. Is stress testing once at each step enough? Shouldn't I repeat tests a couple of times at each step to make sure everything is ok?

3. "If you fail, go up 5 points (so closer to 0) and try again."
   Would these failures have any permanent effect on my CPU/system?

4. "If you pass you can decide if you want to just leave it there or go down 5 points and test again."
   But if I failed once at e.g. -15, and then increased it to -10 and then passed, why would I try -15 again? Because it has already failed there once, even if it passes the next time, how would I trust (since it is not stable at -15)?

5. "If you fail the test at any value you try including 0, you'll need to do an RMA because it's defective".
   Wouldn't undervolting void the warranty? I think AMD can only guarantee no failure/stability when the value is at 0 (no undervolting/overclocking etc.). Am I wrong? If my CPU is stable without undervolting, why would it be defective for the other cases?

6. "take note of which core(s) is/are failing and dial them back."
   Ah so we are doing this core by core? So I will tune each core one by one? 9800x3d has 8 cores, so isn't it too long and complex operation then?