21 years, wow!
How will you be bulk aging?
Barrels? Bottles?
Corks or flip top with gaskets?
Or airlocks?
Just curious.
From my experience, spices will change over time, meaning when you decide to pull them, the flavors will be strong, some spices more prominent & potent than others, but as it ages, sometimes the forward spices will take a backseat & some of the others will come forward. I wouldn't go as far to say the alcohol would preserve them.
My personal experience with clove has taught me to be cautious, very cautious, as it is an extremely strong spice that can easily overtake many others & possibly mask them altogether. (I've settled on 1/2 clove/gallon for my personal tastes). A little goes a long way with cloves....literally.
Whichever way you decide, I'm sure that after 21 years, if everything goes as planned & no issues arise, that will be plenty of time for all of the flavors to play on each other & meld perfectly into a damn tasty beverage.
I hope this helps you.
Happy meading