This forum is mostly 6v6 players so I don't think you'll find much helpful advice here.
I can't give you advice concerning mechanics. For that I'd sooner point to places w more active spy mains (unironically reddit might actually be better for this than TFTV)
Biggest advice I can give you though is to practise in pubs, not on offline scenario maps w low ping (fakelag also doesn't really do the trick imo) and bots that mimic your movement
Experience in pubs is a much higher priority than trying to brute force the mechanics.
As you learn to read real situations, you learn to set up trickstabs to be less predictable and easier to execute.
And over time the mechanics will just kinda get there
In my eyes this video is the equivalent of grinding kovaak's all day to get better at scout or whatever. It's a small piece of the puzzle
edit: the one mechanical thing I did notice is you went for some attempts where you are still in front diagonally of the engi and you kinda just try to stab him in the shoulder hoping it works. You need to stand right beside him for that to work, otherwise your knife always lands on the front 180° of the hitbox