You can also use an 8mm hex bolt in a slotted hole for adjustment, needs a washer to provide a shoulder for a solid fit. That is basically what we use on our idler mount with some added lathe work to eliminate a speed ring and make the assembly a little more narrow in our more recent iterations. Belt looks really tight, it needn’t be. The extra tooth engagement allows us to run loose belts without stripping belts. Running loose belts with an idler provides a little more efficiency and longer belt life.
A smaller bearing should give you the slack you need since your idler is stationary. Nice to see idler’s become more mainstream. Wont be long before every mount on the market has one!