Why not just enjoy the 2.1-2.55 content at your own pace? The point of the expansion is to give people who finished everything new things to do, be it story, trials or whatever they enjoy. But you still have tons of dungeons, trials, etc. left that you didn't do yet, even if you aren't interested in the ARR story at all. So why not do these first? Heavensward is build on vanilla ARR, so of course you are supposed to do ARR stuff before you start with HW. It's not a standalone thing like in many other MMOs, the expansion is a part of the ARR main scenario and NOT independent.

Also, the time investment isn't that big. It's not 10 sets of quests (no idea where you have that number from) but 5, each one can be played through in like 2 hours, especially if you skip the cutscenes you appearently aren't interested in anyway. That makes it 10 hours or something? Even with only one hour of playtime a day, you'll be finished in 10 days. Doesn't seem like that big of a deal to me...