This is an excellent drink but slightly a bit more vermouth flavor than I prefer and I have my bottle of Amer Picon I bought out of Belgium during Covid. I keep it refrigerated and seems ok still.
We have a few wine fridges that keep everything inside at a constant temperature of 12°C. Our rule of thumb is that if it's below 20%ABV then it goes in there once opened. Since Amer Picon is 21%ABV we leave it out on our bar...our bottle is probably approaching 12 months and appears to be holding up well. But maybe we are a bit more cavalier than others. At the end of the day, our humble advice is just to drink the stuff...that's what it's for. Cheers.
I too have a bottle of Amer Picon bought on trip to Europe and shared your concerns regarding longevity once opened. What I now do with all my "perishable" liquids, e.g. vermouth, is not only keep them in the fridge, but use argon gas to replace the oxygen in the bottle. I use a Bottle Thief system in conjunction with a Coravin device. Clearly excessive and OCD of me, but while it may be my imagination, I really do believe it improves the life of these lower alcohol bottles!
We made with Cointreau - and skipped the water
Brooklyn is in the same family - but much smother than a Manhattan
This is in out top 10 whiskey cocktail