I can’t figure out what else I need to do in order for the dimensions in Layout to show up in Imperial units instead of metric. I’ve changed the Units to Inches and Fractional in Document Setup, but when I draw the dimensions it still shows up with metric units
You need to make that setting in the Dimension Style panel. Document Setup>Units is for paper space dimensions.
What you have:
What you need:
Select the existing dimension and change it to Fractional in Dimension Style. Then get the Dimension tool and do the same for dimensions going forward.
You started out by choosing a metric paper size (A3) template so by default the Dimensions are metric. Note that the paper dimensions and margins are shown in imperial units because of your change in Document Setup>Units.