yeah the drops are random , and the RNG can suck balls sometimes especially with Lucky's I have had play through were I only saw one and the next play through every one of my crew had one and I sold 5 more. the RNG is whacked in Evenicle 2
I put on a podcast & spent an hour testing
For drops I'm pretty sure what's going on: 0% for 3 fights, then 20% afterwards. This count is stored in the save. I don't know if "drops" like gal monsters reset the count
For skills, the data I got is consistent with the same thing, but it's definitely way more than 3 fights & way lower than 20% (If I had to put a number: 20 fights, 5%)
There's not enough data to be precise about skills, but I'm now confident that the best way is just grinding weak fights & not trying to be clever
You could, in theory, abuse the cut-off & reroll skills. But the advantage is so small that it's probably better to just take the bad skill. The next drop will take a little longer (exactly 20 fights if my guess is correct), but then there's 1 fewer skill to get in the way of what you're looking for
Test 1
Using a save that had recently got a skill, but had not gotten a drop recently
Do 5 fights, load save, repeat
120 fights, 0 skills. I think about 10 drops (I was just paying attention to skills until later tests)
Conclusion: There's definitely a cool-down after you've just got a skill & can't get another, and it can't be tricked by reloading
Test 2
Same as test 1, but save each time instead of loading (and only doing 4 because it's faster to run back)
got skill after 30 fights (not sure how many had already been done since the last skill, but not many), another after 59
loaded a save, got one after 8 (64 total), loaded, got one after 43 (103 total), loaded, got one after 27 (127 total)
Conclusion: Rerolling does reduce the gap, but not that much, 43 is still quite high
Equipment: most common gap is 4/5, lowest is 4, highest is 14, average 8 (The 14 was on a reload, so it could instead be 10% for every gap between 4-13, or something similar)
Test 3
Test 1 again, but using the save from test 2
This save had 124 fights since the last skill & 0 fights since the last drop
15 loops of 5 fights = 75, 5 skills, 2 equipment
Conclusion: Equipment cool-down works the same way. Skill chance eventually reaches around 5%
imo, the gap between test 1 & test 3 makes it pretty clear what's going on & test 2 shows that rerolling is only kind of worth it (43 would be fine if it gauranteed a good skill, but you could easily reroll the same thing over & over)
This was ~350 fights total, and I got 2 Luckies & 3 Treasure-Mellows. So, both just under 1%. The fact that you got 1 in one run & 10 in another makes it pretty clear there's no clever RNG on those (The skill/drop thing of starting low & increasing makes RNG more consistent between runs)
tl;dr for Skills, Equipment, Luckies & Treasure-Mellows: Just grind auto-battles that you can 1-shot, don't try anything clever
In case you're wondering. Yes, I have been diagnosed