Individual conduits for each phase to reduce amount of stud being drilled
You don't need to make your single conduit nipple that fat

A 2" nominal nipple is overkill here -- a 1.25" nominal Schedule 40 PVC nipple provides 561mm2 of usable fill since the 60% nipple fill rule (NEC Chapter 9, Note 4) applies to your circumstance as your nipple is far shorter than the 24" limit on this rule, while your four conductors add up to just under 533mm2 of area using Table 5 dimensions for the THHNs and a bare 2AWG copper wire for the ground (this saves a bit on fill compared to using an insulated THHN for the ground wire).

Given that the OD of 1.25" nominal schedule 40 PVC is 1.66" as per Cresline's specifications, this fits within the 2.1" maximum bore allowed for a doubled load-bearing 2x4 (3.5" wide) stud by the 60% rule in IRC R602.6.  If doubling the stud is not an option, then the alternative is to use a high-strength (tensile capable/wraparound) stud shoe such as a Simpson HSS2-SDS1.5 with the full complement of specified fasteners to reinforce the bored area.

If you want to use RMC instead

If, for one reason or another, you want to use RMC or EMT with bonding bushings instead of PVC here, a 1.25" nipple will suffice as well, given that you only need a bit under 500mm2 of fill as the nipple becomes the grounding conductor in that case, whereas a RMC or EMT nipple provides over 590mm2.