Alright, just an update.

The shudder is getting much better, either because I have been slipping it so much it has bedded in or I am becoming a better driver. Hopefully it is the later.

Took it back to the mechanic and all hydraulics are working fine. Pedal isn't spongy and it engages in the middle. Was given an explanation about plates sticking, not disengaging properly and it should get better after a while. (didn't understand a word since I am mechanically challenged) Worst case scenario, he will replace it if the clutch is the culprit.
Anyway, i realised the problem exist mostly if I try to shift out of the first gear above 3000rev and after the car has been warmed up (not consistent as it works perfectly fine sometimes). I also have problem shifting from neutral to first and I am currently getting around it by slipping into second gear before I go into first (is that a common problem). It feels like the gearbox is the culprit but I never had any problem before the new clutch and the incident with the old clutch (bearing broke and one of the ball flew out).
Anyone has any idea?

using the Perth-WRX mobile app

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02STi - Vipec, Oval Varex, APS CAI with K&N Filter, Walbro 255L, 53MM Koyorad & STD Coilover

19.5psi tune 271.6HP@6000 351.3NM@4700