Short answer about the direct question QR141 vs 135: installing 3mm washers on each side will also means that the axle will be 3mm shorter in the drop-out. The brake won't be positioned correctly either, and the chainline won't match, as the chainring is positioned for boost hubs (probably not so critical with chains though).
On the idea itself, since you write "looking to buy", I would ask why not buy a bike equipped directly with an Enviolo hub? In this segment, I'm only aware of the Canyon Pathlite:ON 8 SUV or the Precede:ON 7 (if you don't need the suspension) — and many Riese&Muller as well, but these are usually more expensive. The Pathlite costs 1000€ more than the Giant, I would not be surprised that if you count all costs, it won't be so different (hub, shifter, wheel or relacing the wheel, labour,...) and as bonus it has a belt instead of a chain (you need special frames for belts). It also has a suspended dropper post, that is very nice. Note that the Canyon uses the Heavy Duty hub on this bike, that is compatible with 12x148 thru-axle, not the Trekking, so if you want to make the conversion after purchasing the bike, better to take one with this standard, that is in fact very common (141mm QR is quite rare).