Do you have any way to measure the TA of your mead? TA , not the pH. You want the TA to be around 6 g/L of acid. You will need a pH meter and sodium hydroxide. The amount of Na OH you need to add to get the pH of a known volume of the mead to 8.2 is the equivalent of the g/L of acids in the mead. But your tongue is a sensitive instrument and so take a small sample of the mead of known volume and use an eyedropper or pipette to drip in some measured drops of acid solution and taste. When that hits the sweet spot you simply divide the total volume of mead by the sample size you have used and multiply the number of drops by that answer to tell you how much acid blend solution you need to add to the total volume of mead...
Tannin: you might begin by adding about 1/4 t of tannin powder per gallon and let that get absorbed for a couple of days and then taste to see if that helps. Another approach might be to add some oak cubes and rack off after about 2 -4 weeks . I would suggest adding about .5 oz per gallon. Back sweetening may be the most important point, though. Sometimes all it takes is for the mead to have perceptively more sweetness for all the flavors to punch through.
Good luck