Have you tried Royal Rootbeer Extract? Its the best root beer I have ever had, any flavours from the yeast just add to the overall product and aren't pronounced. I do age it for a couple weeks at 55 before putting it in the fridge for a few days before drinking. So it might be yeasty inside those 3 weeks after carbing.
Did you rehydrate the yeast? I don't do this for beer, but I think the large amount of pure sucrose in rootbeer shocks the yeast to much when pitched dry.
I just follow the recipe on the bottle.
1 bottle extract
2 kg white sugar
17 liters of water
1 pack of EC-1118
dissolve sugar in 2 liters of boiling water, add 15 liters of room temp water (going for about 75-80F temp when pitching yeast, i just use tap water its fine here for soda). mix in bottle of extract. rehydrate yeast per package directions and mix in (i think its 1 cup of 95-100F boiled water). bottle in plastic beer bottles (I reuse commercial pop bottles that I oxy clean then starsan). store in dark place at 72F until bottles harden, then cellar for 2-3 weeks. chill well before serving.
I mix everything in a 5 gallon polypin that I dedicated to rootbeer. add each ingredient 1 at a time and close it up and shake to mix (ok when its full i just kinda roll it around a bit). this minimizes contact with possible contaminants and sticky rootbeer scented mess. when its all mixed I bottle using the attached spigot.
Next: Irish Red Ale, Mild, BC Pale, Scotch Ale, Helles, Pils
Planning: Bombardier clone (The real ale cask version, not that sh!te in the bottles)
P1: Chocolate Oatmeal Stout P2: Pear Wine P3: Air P4: Oxyclean
Keg 1: Stout Stout Keg 2: Dead Intruder Mailbock