No one has mentioned this, but in the engine compartment it's preferable to use SXL or GXL wire with cross-linked polypropylene insulation (XLPE). This insulation is tougher and has a higher temperature rating than the more common "primary" wire sold in auto parts and hardware stores, and most likely used in kits like the one mentioned by the OP.
American Autowire, Painless, and other reputable wiring harness vendors use only GXL and SXL wire depending on wire gauge.
Good explanation here:
https://www.delcity.net/store/Cross!Link-Wire/p_804928
You can buy shorter lengths from Jeg's and other vendors. I used Jeg's 8-gauge SXL wire for my last truck's charging system and fuse panel feed.
Of course, the best path to brighter headlights is a quality wiring harness with 1-2 sizes larger wires than stock. In addition to much brighter headlights, you'll have brighter tail lights, turn signals, and dash lights, as well as more voltage to the ignition system. Heck, your heater fan will even blow more air, and windshield wipers will run faster!
But if a new new harness is out of the question, you can always use more relays. I'm just looking for a reason to buy something like this 4-channel solid state relay.
https://www.speedwaymotors.com/MSD-7...el,342137.html