Theguysayhi- I don't think your spacing was bad necessarily it just didn't account for things like hurtbox shifting. Like, in two of those clips you tried punishing Yoshi's Fsmash but since Yoshi throws his head forward, part of it connected with the sour-spot hitbox. That happens quite a lot, actually. You think your spacing is perfect but your opponent's limbs are flailing in a weird direction and it messes up your tipper hit. Sometimes, that's impossible to avoid. The way Marth's hitboxes work is that the sour hitboxes outrank the tipper ones. So if say the tipper hitbox connects with the body, but a sour hitbox connects with a leg sticking out, you'll get a weak hit. That also happened in Smash4 but in that game, since people couldn't move as freely and since some flinching and tumbling animations were different, it wasn't as much as an issue. Also, having a guaranteed tipper out of jab means you didn't have to depend on stray tipper hits as much whereas in this game it really stands out.
One thing I also noticed is that you like to punish whiffed moves by dropping shield and either Ftilting or Fsmashing (or at least you did in the clips you showed). In general, I avoid that as much as I can since dropping shield adds 10 frames to whatever move you do (plus a few from hitstun). I've gotten into the habit of either doing an aerial OOS or Uspecial. This is especially true online since the lag and latency are so horrendous that more often than not I end up whiffing my Fsmash and I end up getting Fsmashed myself since Marcina's smashes are EXTREMELY laggy. Even if I do use a smash I'll either use Usmash or Dsmash. Usmash cancels shield and it's just 13 frames. Dsmash doesn't cancel shields but it's only frame 6. Now, some might say "well Fsmash is only frame 10 so it's not that much slower". But the thing is, Fsmash IS frame 10 but it starts ABOVE Marcina. It hits an opponent in front of you on frame 12 or 13 (depending on opponent height and spacing).
So Uspecial is your fastest OOS option at frame 5, Uair is next at frame 8, Nair and Fair are frame 9, shield grab is frame 10, Usmash is frame 13, jab is frame 15, Dmash is frame 16, Dtilt is frame 17, Ftilt is frame 18, Dancing Blade is frame 19, and Fsmash on an opponent in front of you is a whopping 22 or 23 frames.