First of all, it looks great.
My only concern is related to the “default enabled”. I think that it should be enabled or disabled by default the same as other areas of Blender. Of course, lately you should be able to save your defaults for the VSE.
The state of defaults is not conclusive. Inconsistency has been pointed out and discussion is followed in T89665
We thought that with snapping being not as intrusive would allow to enable snapping by default. I also wanted to hear feedback from users before final decision. In any case the UI inconsistency argument is strong and as you said, there is ability to save startup file with snapping enabled if you wish.
When transforming a handle position and an effect strip is applied(ex. speed) it’ll snap to the original handle/(effect strip?) position, making it impossible to add a few frames by dragging the handle.
Snapping Playhead to keyframes as an option would be a very useful feature, when working with keyframes in strips like the speed strip keyframes.
This happens, because it is snapping to effect start frame. That is a bug.
But I wanted to fix snapping to hold offset even if it is 0. This way you can just grab handle until it snaps so you can eliminate still frames. So this will be similar behavior, but I think annoying thing here is, that effect causes original position to be “sticky”, which wouldn’t be true with 0 hold offset snapping.
I will write this down to check, I fear this could cause performance problems in heavy files. But this option could be disabled by default in such case probably.