I started soaking my pepper seed in salt peter solution of few years ago. I have no idea what the "science" behind it is (if there is any
) but, at least for me, it has greatly improved my pepper seed germination. Just soaking them in water for 24 hours helps a bunch. I had roughly 1300 seedlings and only 30 cups that did not germinate. I also believe, and again, this is just me, that pepper seed likes it a lot warmer germinating than we think. I set all my flats on a "heated mattress pad". The containers have lids on them. I left a thermometer in one to see how hot it got and it was 92 degrees. All but one cup in that flat germinated.
There are lots of variables as you said, age of seed, USPO Trauma, etc. Fresh seed (seeds saved from the months before a new season starts) is a big help although some members here did a "test" of ancient seed (some was 20 years old) and were amazed at the germination they got.
Pepper seeds really are a pain in the derriere to germinate compared to tomatoes.