Andrew, your plan is fine for what you need to accomplish, IMHO. You could even drop down to 6" sooner if you need to...it will sacrifice some performance, but 6" 26 guage is easier to source than 8". But check with the local HVAC suppliers...many folks have discovered they could get spiral pipe for not much more than snap-lock. And whatever you buy in duct now should be re-usable once the new shop is built. KenCraft reportedly has good pricing on the wyes and other components...you really cannot use HVAC wyes for this application...they're backwards.
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The most expensive tool is the one you buy "cheaply" and often...