Re: Line from the hardware store?
Halyards don't need to be comfortable. On most dinghies a downhaul or cunningham with a multi-block purchase is used to adjust luff tension. The halyard is used once to put the sail up and not with tons of load.
Dinghy rigging is quite different from keelboat rigging. I didn't realize quite how different it could be until sailing on Lightnings and 505s. As an example on 505s it is common for the vang to have huge loads (and use a cascading 24:1 or higher purchase) while the mainsheet has a very low purchase. When close hauled the skipper sheets with the vang, not the mainsheet.
I haven't sailed on a FJ and don't know how some aspects are rigged.
The masts on most dinghies are also much smaller and more flexible than what you'll find on a keelboat. There just isn't room for a 5/16" or 3/8" sized halyard.
The only 3/8" line that you'll find on most dinghies (and again, I've never sailed a FJ) would be on the main and jib sheets.
boats that I sail on regularly: Pearson 28-2, Rondar 505, Yankee 30, Blanchard Junior