You can simplify things hugely by simply having a separate and much smaller cabinet for vegging. That simplifies your ventilation, your lightproofing, everything. And you will make use of the extra space in flowering, I guarantee you. If that's not an option, and you must have it all in one cabinet, then yeah, it's gonna be a pain in the ass.

Envirolights do give off heat, btw, so you'll need some ventilation for that, whether it's in the same or a different cabinet. If it's in a different cabinet, you can get by with computer fans and/or axial fans or fart fans, or whatever's easiest for you to get; you won't need a 'real' (centrifugal, squirrel) fan for that area.

I totally understand your desire for a one-cabinet solution; I wanted to do that too. But the more I thought about it, the worse it sounded, so I ultimately went with a separate smaller veg cab (and another for mothers too), and I am very glad I did it that way, no regrets whatsoever.