Best Animation Pipeline for creating completely custom animations

Hi people! I’m trying to create fully custom animations for my game. What I have been doing so far, that have worked, is:

For example, let’s say I want to make Door opening animation.
I get the player in correct position and rotation. I copy the mesh current position and rotation.

Then, I open a sequencer, based on idle animation for player character. I put the copied position and rotation there (in front of a door).

I then edit keyframes, bone by bone, to achieve what I want. Export it as an animations sequence, make a montage out of it and play it when the door is supposed to be open. I also adjust it, so it’s aligned with the actual door being opened.

However, this does not always get me correct results. For example, I make the animation perfect in sequencer, but when I play it for player character mesh in the same position and rotation, it can be slightly off. Not sure why is that happening.

Overall, do you guys have some other way to handle stuff like this? What would be the best animation pipeline to use in this kind of scenario, that can give out consistent results?