So my experiences with these moves. I can't really give you any specific tips because I would have to literally see what you are doing to see what is going wrong. Because if the technique is wrong then everything else you do becomes kinda irrelevant. also you guys are talking about degrees and yeah they do that in the videos too, for me that does not work at all. I have no clue what degrees I'm holding it at. But I just keep at it till it works because it becomes a feeling and muscle memory that works better for me.
First thing I learned was to do a perfect dribble, if you cannot do this then you won't be able to do a perfect anaconda or riffle fan.
And trying to learn those moves with a half mastered dribble is a bad start because you will just make it harder on yourself.
Having a new deck for the riffle ONCE you master it definitely helps because it runs much smoother but it can also go too fast much more easily. I find that most of my old decks still work pretty well for the anaconda, though again a new but broken in deck will be better yes because the suction effect is stronger then with an old deck. It took me about 3 weeks of practicing the anaconda every day to get close to a straight line dropdown and it's still around a 5 out of 10 times perfect thing but at least cards aren't dropping anymore

So I don't know how long you've been at it but it can take a while varying from person to person ofc.
I feel like you guys are making it harder than it has to be a little bit. All I did myself was watch the video of the original creators and from there I just started practicing and never watched a video about it again. I like to do it like this because then I learn how to feel out the mistakes and improvements myself. It might take a bit longer but in the end I will understand the move and the physics much better.
Also the Lepaul spread is a thing that's been bugging me too haha I feel like my deck is unusable for it after only a few tries and I'm hate to break open 10 new decks just to learn this move

But damn I want to learn it though because that 1 handed fan you can do after you learn the lepaul is freaking awesome.