First off, the “art” you’re speaking of for hitting combos, you should have this nearly 100%. Think about it this way, would you ever want to drop or miss your combo? If you do, then fighting games is something you shouldn’t be pursuing. It is just like anything you do. If you want to be a basketball player, would you only want 3 out of 5 of your shots to go in or always have it 5 out of 5? It’s the same mentality here. Always practice to make sure you get your timing right.
The “xx” part of the combo you listed means to cancel the button before into the move. For the combo you posted, it says cr.MP xx H Hadouken. That means as soon as you throw out your cr.MP, cancel it immediately into the move, a heavy Hadouken.
Spacing is insanely important for combos and just in the game in general. Not just SF4, but all fighting games. Certain combos are spacing and character specific so knowing what combos work on who and where is important. Having a fighting stick based on the idea of making combos easier or not depends on the person. Honestly, use what is the most comfortable to you. I grew up playing Marvel back in the arcades so I have to play any Marvel game on a arcade stick. For all other fighting games, I play them either on a PS2 pad or on keyboard. It is really just preference. There are some helpful things about playing on a fightstick or pad too though. Like an example would be that plinking is a lot easier to do on a fightstick (at least for me and I would say most would agree too). Plinking is a technique where you can hit tigher/stricter links much easier. It consists of you drumming through two buttons to ensure links come out easier.
The way I see it, if you hit a ceiling, that is a good thing. That means you can get better. Watching your own matches is probably the most important thing. Watch what you’re doing and what decisions to make and figure out why you did it and why it made you win or lose. Blocking is important, definitely. Rather than just saying blocking, I would rather say improve your defense in general. It is not just about blocking but evading as well. This all ties to defense. Backdashing is a great tool to dodge and get out of situations. The thing is, relying on anything can be a bad thing too. Backdashing is like a double edged sword in this sense. There are option selects/setups that make backdashing useless.
Combos you should be working on are damaging combos and situational combos. The trial combos are good building blocks for your in-game combos. FADC combos are also important because they are probably the most reliable way to link your Ultra’s into your combos. Rather than just practicing combos, practice your neutral game/fundamentals. Practice things such as your spacing, footsies and tendencies. Jumping is one of the worst things you can do in SF4 because you know; you can’t block in the air. This is why dragon punch moves are so dominating in this game.
Ryu is probably one of the best characters to start out with because he is pretty much the full package to learning the game. You learn the projectile game, spacing and you have a DP and so on… You’ll know when you should be move on. If you solidify your neutral game, that is when you should start moving on to another character. Since you now know your basics like spacing, zoning and so on, you can bring those same teachings to other characters. After you get your neutral and fundamentals down, it is up to you to find what character/style that fits your gameplay.