A week sounds reasonable. The custard is going to be the foot-dragger... and it is not too bad as far as custards go. And, as FranC already mentioned, there is no restriction saying you can't do test vapes early.
I will often do test vapes, early and often; for concentrates, of flavor profiles, that are unfamiliar to me. I keep my expectations minimal (not expecting "instant winners"). But, instead, try to learn how the flavor(s) develop. It can often highlight different flavor nuances as it matures... some fading, others becoming more pronounced. Some do take some time to appeal to me. Others, I discover I actually prefer younger/fresher, and less as they mature.
I find repetitive testing can be informative if I am willing to let it.
I guess "plain" is defined by one's own taste buds. I find it to be a very good rendition of its name: a shortbread biscuit (cookie), just the way it is. I respect that we all taste things differently... and a plain cookie may not be your thing.
That said, I could also see it being a good "stone" (shortbread cookie foundation), to add fruits, jams, custards, and creams too. Maybe even use as the "crust" component for a pie, cobbler, or cheesecake.
Fran, I know you like to jump straight to the complex. But, if you are going to have the ingredients out anyway, I would encourage you to mix up a small (5-10mL ?) tester just as written, along with the mix including whatever additions you are contemplating.
You may have been thinking of the YWCC
reviews. I have seen more than one prolific reviewer mention that it only requires 3(ish) days.
As I mentioned above, I think it is the custard that is stretching this out to a week (which is still pretty good for custards in general).
If you found it too rich; I am not overly optimistic that more time is going to help. In my personal mixing experience, I find richness tends to hold steady, or increase with age. If this second attempt fairs no better, maybe think about cutting back the custard by 0.5-0.75%
I have learned, for my own personal tastes, that custards are often too rich almost to the point of being "heavy." Of the different custards I have worked with (TFA, CAP v1&v2, FA, and INW); INW is the least rich/heavy. But, I can see where it can still be overpowering... especially in combination with cheesecakes (which share a similar problem for me).
