I only ever resteep when I am brewing gongfu, that is multiple, short steeps with a lot of leaf and a little water. I am one of the few people who never ever resteeps when brewing western style; they always seem weaker and less flavorful to me no matter what and I feel like with western brewing most of the good stuff comes out in the first steep anyway.
If it helps you can think of western style as a whole bunch of tiny gongfu steeps put together in the same cup (which is sometimes how people brew tea in China). By drinking each one as you go along you taste different things at different times, versus all mixed together in the same cup, which is the kind of flavor profile you get from western style.
I can usually get two 12oz mugs out of all varieties of Twinings and similar CTC loose leaf teas without a lot of loss of flavor. Better China full leaf blacks, I can get 2-6 depending on the tea and my mood. Try adding a little of your favorite sweetener as the flavor diminishes. That might bring it back to life for you. Greens and whites I average 2-3 steeps (some go more). Oolongs 4-6. Puerh, I usually quit before it does.
I do follow directions unless I don’t like the results. Usually it calls for 1tsp per 8oz so I use abt 1.5tsp. I usually use near the minimum time listed for first steep. Maximum listed for second. I judge later steeps more by color than by time.
Oolongs and puerhs are different. I use a longer first steep, then cut way back on the second. Adjusting from there for later.
Something I didn’t see mentioned: a teaspoon isn’t a reliable method of measuring tea amount for ounces of water.
Depending on the size of the tea you can get hugely varying amounts of tea. In fact, there are a lot of teas you can’t even measure in that way as they won’t fit.
I’d suggest going by grams instead of a teaspoon. I use 2.25-2.5grams per 6oz of water, personally. That’s a common ratio. You can use any amounts you find you like. When you go by grams rather than trying to go by different amounts of tea per teaspoon you have a better standard to go by, imo, for tweaking what works for you.
Verdant’s Laoshan Black can stand up to your method; it’s pretty much what I do. I followed the directions from the web site—near boiling, 3 minutes, 1 tsp per cup—using an infuser basket right in a tea cup, and then kept resteeping as needed using the exact same directions (I did start letting it steep longer about round 5 or so), and it resteeped beautifully. I’m with you most of the time; it’s rare I find a black tea that tastes even almost as good resteeped even once.