Optimum height is the minimum height that will still allow you to work with the thickest stock you will regularly use. If you expect to occasionally use thicker stock, or work on odd-shaped pieces, consider a removable/drop spoil board.
Remember that CNC is limited by bit length. In general, you can only carve as deeply as the bit sticks out of the collet. So having an 8" Z axis only lets you carve on the top of thicker stock, not any deeper. Even if you can source extra long bits, these machines really aren’t designed for that sort of work. The forces working to deflect the bit get pretty bad when you have a lot of stick-out like that. The MPCNC (and really, any CNC machine), works best when the tool is as close to the gantry as possible. Try to shorten as many of the lever arms as possible.
And yes, 55% infill is plenty. Assuming you have a .4mm nozzle, and run somewhere around .32-.38mm layer height, three perimeters and three layers top and bottom should be enough. If you are feeling paranoid and have the filament, up the perimeters and layers before the infill. 55% is plenty to keep the piece structurally sound. The most gain you’ll have is from thicker walls, including the top and bottom.
When you go to assemble, please keep in mind that the pieces are designed to clamp the conduit, and there are supposed to be gaps. Do not try to force all the pieces to meet (you will find out how your prints fail). And pay attention when the instructions say to only engage the nylon of the nylock nuts. It’s easier/better to start with everything loose (except your grub screws, break out the Loctite for those little buggers), and tighten in small increments as needed to bring everything into square.