It's not easy to critique CPU matches. Ideally get other human players involved if you can.
I would practice L cancel timings and I would practice optimal flowing movement from throws and attacks. For L cancels, the main things you should understand are as follows:
1. You can press the button for L cancel multiple times if you aren't sure about your timing. There is no penalty for "spamming" the L Cancel input. It's actually possible to "spam" the input in a controlled manner so that you stop once you notice the L cancel succeeded. At the very least, that is a back up option to consider if you keep failing L cancels.
2. L cancel timing has the same window of input before you land, however there are things that change how quickly you land. Fast falling is the obvious one, but the not so obvious change would be hitlag. You get hitlag from hitting the opponent with one of your attacks. If you L cancel right as an aerial first hits, chances are your character will be in hitlag and you will fail the L cancel window. I can almost 95% guarantee this is what commonly happens to players. They get used to L cancel timings that do not involve hitlag, and then once hitting a person they mistakenly press L cancel too early.
3. Optimal movement from throws means understanding the exact timing you can first react, or move, after completing a throw or attack. One quick way of figuring this out is to hold shield after doing a throw. Your character will shield basically as fast as the game allows, and naturally if your character can shield quickly after a throw, they can run or jump instead of shielding. Practice timing your run or jump after the throw, while taking notes on how Captain Falcon's body and animations look while he recovers from lag. When he Up throws someone for example, he kind of punches them upwards and then his body returns to a more normal stance. If you see that, along with getting a good sense on how fast his shield comes out afterwards, you can substitue shielding for using a jump or run at the same timing shield comes out.
3A. The trick to this however is understanding how Melee buffer system (or lack thereof) impacts you. One thing people commonly fail to realize is that Melee will basically ignore your inputs in many cases IF your character is still suffering from cooldown / lag. That means you have to find out what a move's cooldown is, and you usually have to respect that cooldown. Trying to move, jump, grab, etc before that cooldown will not matter.
This explains why if you throw or attack someone, and go to quickly dash for a follow up, your character may instead walk forward or not even move at all. Also, if you keep getting Up Smash instead of Uair when you are trying to combo, you are probably pressing C-stick or Up + A too fast. I think you were cancelling jump squat too early going for that and getting Usmash instead.
Becoming good with Falcon, heck maybe most characters, comes down to an intimate knowledge of details like that being learned and applied on top of other fundamentals. It will become easier to critique you after working towards that, and getting matches with other human players.
