Strength (mostly finger, I think, but some wrist), experience, callouses, and “touch.” Now, Syn plays 10-52 strings, but he plays FOR A LIVING, and so even though I envy how much time you get to play, he gets to play more. I tried 10s, and ultimately settled on 9-46 (lighter top, heavier bottom) for my strings.
I do not believe fretboard wood plays into it, though some insist fretboard RADIUS does – I don’t notice it. Most of my fretboards are rosewood, but I do have a maple board, too, and I don’t notice a difference in bendability.
The last video I watched of you – the 4 clips starting with Scream – I noticed how much your vibrato had improved, so maybe you’re being too hard on yourself?