I was double checking some of the measurements of my parts compared the the specifications you list, and noticed the 120mm AIO width is a hard limit, having only 120mm of clearance. Looking back in the forum posts from earlier this month, I remembered you guys said that this was set in stone as it was a function of the chassis. However, limiting your clearance to exactly 120mm cuts the number of compatible AIOs down fairly significantly.
For most people building an aesthetics focused SFF rig where the AIO is visible, options matter a lot, and a lot of LED focused AIOs and premium custom blocks have slightly oversized radiators. The Ryuo was mentioned earlier at 122mm, and I think this (or possibly 124mm) is a good sweet spot for fitting the vast majority of premium coolers.
This case is first and foremost about aesthetics, and limiting your choices of premium AIOs because of 1-2 mm of clearance seems kind of silly (as a hobbyist case modder this is the kind of thing I would break out the dremel for).
Edit: In regards to how to accomplish this without changing the front pillars, would it not be possible to simply add 1m to the outer wall thickness and then cut 1mm of the inner thickness up until the front pillars (i.e. have a small lip on the inside where the rest of the case meets the pillars)? This would require a simple tweak to the front and back panels of the internal frame, just to extend the edges by 1mm.