I'm not that great of a marth player because I started not that long ago, so my critiques might not be the best. But here goes!
First, about your dashdancing, it seems that most of the time you DD is when you are further away from your opponent. When the opponent is a little too close for your comfort you start doing something I do all the time: roll (this is a habit i'm trying to break). It is a competition and you might not be super confident with your DD so that's why you go to the roll instead (i'm the same way... not confident with my DD at all). Just try to stay mobile and DD->WD or WD->DD (I believe someone called it "surf wavedashing"). It gets you out faster than rolling and you can control your distance better and act out of it faster. Rolling makes you vulnerable to punishes because the length of the roll is set and your opponent can read, react, and punish accordingly.
Getting back onto stage: On your 2nd stock in the FD match(11:00), when you got sent out to the right side, you used your double jump and did not side b until you were almost level with the stage. You definitely could have made it back there. Also, try to save your double jump to trick your opponent and add some mind games into it. If you still have a double jump, you have more options. For example, your 2nd stock on game 1 (7:30), you double jumped-> side b-> up-b-> got hit-> died. If you saved that double jump, you could have went for a 2nd attempt at getting back to the ledge. Start Side-Bing earlier and save that double jump! Now, this is a tip for getting back to the ledge. Now onto options from the ledge... You can waveland onto stage like you mentioned. You can also ledgehop->nair/fair to cover (if your opponent is too close to the edge). You can air dodge onto stage like what your opponent tried doing in game 2 on his 2nd stock. You can roll, jump, get up attack, and normal get up. Just know your options and mix it up based on your opponent's positioning. You are at a disadvantage when you are being edge guarded so don't expect to be able to make it back safely 100% of the time. But if you mix it up and make your opponent think alot, then you'll be able to make it back safely more often because you are unpredictable.
Ledge stalling.... You can ledgehop->fair->regrab ledge. You can hax dash (it's hard.... ledgehop->waveland backwards->fastfall->regrab).
When your opponent is above you, try to use more utilts/uairs to juggle them around and rack up damage. Like you said, you fsmash too much which leads to getting punished. If your opponent knows you're always going for an fsmash, they can prepare for it and counter accordingly. Habits are hard to break so just keep practicing.
The biggest thing I think you should work on is the edge guarding. I did not see you grab edge that often and the edge was pretty free most of the time. What would help is knowing how to get to the ledge. You can......
1) short hop->bair (turns you around) -> grab edge
2) wavedash backwards -> fast fall -> grab edge
3) when facing edge: dash forwards -> DD back -> wavedash backwards -> fast fall -> grab edge (I think some people say this is faster than turning around and then doing #2 but you have to be fast with it)
You can then refresh invincibility or roll onto stage or get up attack. You will have invincibility frames and your opponent can't grab ledge until your roll/getup attack animation is completely finished. So even if they sweetspot, if you are in one of those animations, they cannot grab edge. You can ledge hop bair to knock them further out (if they come too close to the edge while you're hanging onto it. You can ledge hop dair (onto stage) for the spike (if they really come WAY too close to the edge while you're hanging onto it). If they upB onto stage, you can react and hit them (and maybe start a combo or hit them back off stage) during the landing delay. Marth has a great edge guard game so take full advantage of that. For example, at 10:55, you got hit off the ledge by your opponent and he was able to grab on. If you rolled, he would have died.
When your opponent is in front of you, try to use some jabs or ftilts. They don't do as much dmg or have as high of a knockback but they're much less laggy.
You stay grounded quite often which is something i am trying to work on. when you are under your opponent, you have the advantage which is good. Your dtilt game and spacing is better than mine (I always forget to use this... low lag, good range, and hits them before a smash or something can come out cause of how fast it is).
I think just knowing your options and mixing them up would help you alot. This is what I can think of for now, hope it helps!
