Hardest stuff was finding a fucking 3 star moose
I recently finished RDR2 after putting it off for years and seem to have gone the same route you did, to finish up on stuff outside of the "official" 100% - It's a little surprising just how much content is not included in the official in-game 100%.
Agreed on the gambling challenges, I'd read about them and was fearing the worst for gambling 8, but it was just time consuming, tedious repetition, but not difficult at all. Gambling 9 put me off at first, but I did some quick reading and learning how to play, and did the challenge pretty quickly. Especially after moving from block to all fives, at least you can score during the round. All the other challenges, gambling and otherwise, were either piss easy or very briefly annoying at worst.
I didn't mind the small bird hunting after a little while, actually began to enjoy the challenge - for the most part. A couple-few of those little shits really like to hide. But otherwise I got clued in real quick and MOST of them can be almost anywhere, with some locations outrageously better than others. Just needed to have a keen eye and listen closely.
The moose were pretty bad for me for a while. I got real familiar with a bunch of the spawn points, but on the rare occasion the game graciously spawned one for me it was always two stars or less, started to become fucking enraging. Worse, the Cairn Lake bait trick, which I'd read about in numerous places, did not work at all for me. I got exasperated and started looking up shit on youtube, and what did work for me was doing what this guy did:
It's shown being done in RDO but I didn't even notice that until I tried it in SP. Worked perfectly, got four in relatively lightning fast succession. So basically the same as Cairn Lake but just in a different location. Of course, what worked for me might not work for someone else.
I don't know if it's true but I've read that in addition to how things gets shuffled around after 72 in-game hours, or save reloads, or quit to menu and reload, etc, is that when you start an entirely new game some spawn routines are decided. Which could be among the reasons why there’s just as many “there ain’t shit here bro!!’ type posts right alongside the “got it in one try, thanks!” Who knows, though. I watch the following video and I have no idea how this guy is doing this unless he's either editing out massive amounts of time played or if there are many in-game days between each location.
Even so, getting so many is just absurd to me, nothing remotely like that happened to me at any attempts in any of the locations I tried. And although it did seem to come close at my location, but no idea how long that would have kept up.