O.P. Much good advice here, but I can chuck in my couple of cents worth.
I come from the engineering world, where you do quite a bit of tapping, compared to the automotive world.
First off, buying used taps is always a bit of a lottery. I’ve been fairly lucky, but I’ve had the occasional tap that’s blunt as anything and just won’t cut.
So, if you can, start off with a new tap, of decent quality. At least till you get more experience. I note your preference for U.S. made. Good idea. This side of the pond I tend to go for British made, but I’ve used German and Japanese without problems.
I’m generally unimpressed with these big sets, especially those designed for mechanics.. Better to get individual taps to start, and keep costs down by just buying what you need. A good source is MSC, if you haven’t got a copy of their catalogue it’s well worth a call.
Don’t get too hung up in HSS v Carbon steel. The main difference with HSS is that it retains it’s temper at higher temperatures. For hand tapping it doesn’t matter. Some folks insist on HSS, but by going for it they usually end up with a better ground tap than they were using, and that’s the difference!
Some materials can be hard work. For anything other than brass or the usual steels I usually consult a specialist. I have one I can ring, but again, for a big firm, I find MSC are great at this. I’m not familiar with hot roll flat bar, but wondering if that’s getting tougher for tapping?
What size tapping drill are you using? In my experience the ‘book’ sizes are just too tight. Opening up the hole just a little makes all the difference. (A firm I worked with were breaking taps at a horrific rate putting 4mm taps into a 3.3mm hole. Switching to 3.4 made all the difference)
I assume you are tapping by hand. Use some lube (proper cutting lube is good - MSC again) but remember it’s lube not coolant you need.
Starting with a new tap, you will develop a feel for when it’s cutting properly, and when it’s getting blunt. Tapping machines used in the aerospace world measure the torque required and switch taps when it gets too high. You don’t need to be that accurate, but you get the idea!