You’re using full block so you should shift at all times but you should probably use half slab instead

Edit : oops I saw wrongly that it’s actually a slab

Perch is actually RNG, so good luck. But you can increase the chance by standing near obsidian tower, and the chance will greatly increase if you tower up to y: 123

just use obsidian to do the same thing in ssg its easier faster and you can do a 4 bed pretty easily

you can also use slab and water log it, it works the same as obsidian as it does not break when the bed is exploded (useful for 1.14 and 1.16 if you somehow dont get obsidian/cryingobsidian)

You may have waited too long, when the dragon gets close to fully perching, it can fling you away.

After reviewing the video, it looks like you got a diagonal perch, which is unlucky because it looks a little faster than the sideways one.

Also, you missed a bed.

The reason you get pushed is because your hitbox comes in contact with the dragon's hitbox, and when you get pushed into the air, when you went back to the dragon, it was already fully perching.

the dragon will push you back in you misstime the bed explosion, the way once cicle works is basically a stun lock in the dragon, if you misstime a bed and jump the dragon will hit you, the way people does it now is by using obsidian or crying obsidian or else a waterlog slab at the side of the first air block over the bedrock pilar, if you want, dm me in discord and i can give you some tips and a link to a practice map.

just miss timing hits plus it was diagonal perch stand head on with the pillar east-west and itll perch straight