Running a Circular saw along a flatbar is a tried and true method for achieving straight cuts.
Some cheaper saws, the blade is not truly parallel with the foot, and it can cause drifting, or binding.
If you do a lot of cuts like this, what is helpful is having a strip of Melamine or thin plywood, about 8 inches wide, and then gluing down a straight edge to it. Run the saw along the straight edge of this guide board, and then in the future, you need not measure each and every time for blade offset, just lay the edge on the marks of the board you are cutting, clamp it down, and cut. Some people glue sandpaper to the bottom of this piece and do not use clamps to make it even quicker.
While making cuts is certainly possible on either side of the foot, it is better and safer to keep the wider part of saw foot on the piece which is not going to 'fall' at the end of the cut.
I would have been scared to take the time to take pictures of the glue up on such a big surface area. That glue does not really give much working time to get all the bolts through and tightened, depending on temp and humidity.
But good job, I look forward to your updates.