I only shoot up to 12x20 and a couple of lenses that I find work very well in addition to the 355 G Claron are the Fuji 600MM and the 24",30" and 35" Red Dot. The longer focal length Dagors are very rare and exceedingly expensive.
If I was shooting 16x20 and 20x24 the Schneider 550 XXL would be a fabulous choice because I hear that they cover like a big dog right to the edge and are very contrasty. The 122mm filter size of this lens would likely induce migration to an external compendium possibly affixed to the camera and large square filters. Everything operating within this unique domain is costly, but what else is new?