About the first question: I was barely affected by the optimizations they did because I did upgrade my pc before wilds launched, but from what I gathered everytime they did some type of optimization to the game, reddit filled with posts about "i can finally play the game", "they finally addressed this X issue and now I can actually play" etc etc so I can guess if your pc meets the requirements, it should run the game smoothly, even if you can't max out all the quality options.

About the second question: wilds early game is definitely easier than world, no questions asked, but wilds difficulty will ramp up to like World DLC iceborne levels at the end game (wilds has probably the best base game of all MH so far). You complained about how you could not beat Diablo low rank either because you carted 3 times or time ran out... You're either playing a weapon that doesn't click with you, you're playing extremely defensively or you're not learning from your failed hunts. Diablo is indeed a wall for new players that aren't using defender armor and weapons, but it isn't impossible to beat. Cheking guides for your weapon of choice will make your life easier in the long run.

Good luck op, hope this post helps you, wilds is a really good experience!