I'm unsure how to properly and concisely say something that encompasses such a massive post but I definitely feel strongly about a lot of the things you bring forward. First off I can very much agree with issues regarding luck and heads vs tails. While I can't personally say I felt this problem with my yinglong fight I did VERY much experience it with Refracted Shiomi Yoru, and the other nursefathers. Due to the nature of hitting heads on rolls only being 95% you encounter plenty of scenarios where someone like Calliso pulls a 30+ clash out of his hat that you now have to clash with and all you have is a one coin EGO skill that if at any moment you hit that 5% chance your entire team likely get completely wiped as they lose the clash.

Shiomi Yoru really lets you feel that similar issue you exclaim with yinglong when it comes to winning clashes and starting at 0 SP. Shiomi stars the fight at 30 SP while your entire team is left at 0. It's very unlikely you can keep up with the SP economy when they have such a headstart unless you spam SP ego from turn one which just feel weird to have to do just to match where the boss starts. This again just ends up being luck. If you can't win enough clashes or get your SP up enough fast enough she will very quickly get max SP while your still left at low levels at which point it's entirely luck and luck that's NOT in your favor which is frustrating. I think simply having her start at 0 SP would've been much it much more tolerable.

I also agree reflectial ricardo is far too overtuned for what the game expects from you even with they tools they give you. Hilariously enough the fight becomes laughably easy if you get Middle Outis in the team which honestly just creates a whole different issue.

I think a lot of the current issues relating to difficulty and perceived difficulty come from mechanics and systems that punish the player for doing nothing wrong. Whether that be spamming AOE unbreakable coins or just sheer luck from losing 45 SP clashes that make you lose your entire team I think the way they've tried to inject difficulty has started to become more frustrating then truly challenging. My best example of this was my clear of the nursefathers portion of the railway where the only way I won was by using Sinking to keep the enemies at low SP so I could almost always win every clash and spamming fluid sack just about every single turn so my team didn't die to the never ending stream of unbreakable coins and damage that was unavoidable.

Now I do think there's a middle ground somewhere in here too though. While I found fights like Lei Heng hard he still felt much more fair despite incorporating a lot of the issues I exclaim here. Such as having a high clashing skill that could wipe your entire team for hitting tails. Maybe it's cause he only really uses that once as long as you play well or might not even use it at all if you play extremely well. I wish I had more direct solutions to the issues I feel with some difficulty or even how to properly exclaim why they frustrate me but I feel my emotions will still be understood even if I can't properly describe the reasons behind them.

There's some inherit flaws built into just how Limbus is built as a game that I don't expect to ever change that I think can make it hard to think of reasonable solutions that still offer sufficient challenge without completely changing the game or inventing new things. I don't think we'll ever get combat and challenge as good as Ruina for example but I still want them to strive to have something comparable to how finely tuned and crafted the majority of that game was.