Some additional thoughts on adding spices post fermentation.
Adding an extract to the finished beer is the best way to get just the right flavor. But by adding a measured amount, you can record it in your notes and just add it to the kettle next time.
I think you'd have very different effects between adding it post fermentation and to the kettle. If you add it to the kettle, you'll likely get more flavor and less aroma than you would post fermentation. If that's the case, not only the effects, but the amounts, would be different.
+1, also cooked spices taste quite different than raw spices. This is actually the secret to great indian food at home, cook the spices.
be careful with tinctures of woody spices like cinnamon because you can also extract tannins if you leave it too long.
That being said, your best bet post fermentation is going to be a tincture. Or dry spice. The risk of infection is real but not that significant. Use a sanitized hop bag or tea ball to contain the spices, don't grind any of them too fine and drop in primary after fermentation has settled down or secondary if you are doing a secondary.