I would down mix the 54mg Nicotine base with PG:VG to what ever you are going to
vape at, 18mg?, and then try it. If it tastes really harsh then it is your 54mg Nicotine base.
I get my Nicotine Base from xtremevaping.com also and it is not harsh at all. A tad peppery but not harsh.
You could also be using too much flavoring without and sweetener. I like Sucralose as sweetener myself. I also do not start a DIY recipe with more than 10% flavoring. Easier to add flavor a drop or two at a time then it is to dilute an over flavored
e-liquid.