I have kept an imperial chocolate stout on tap for going on 10 years straight now, served with beer gas through a beast of a Micromatic stout faucet. I carbonate with straight CO2 to just 1.2 volumes then dispense on 70/30 beer gas at 35 psi and get a wonderful cascading pour.
To carbonate to such a low level I do it when the beer is still warm-ish: I rack from the fermenters to the kegs without a cold crash so they're sitting at fermentation temperature around ~66°F. I put 7 psi on them and let them sit like that for a couple of weeks, before moving them to cold storage or directly to the keezer. If you look at a carbonation chart/table you almost have to carbonate warm otherwise the volumes go up which will wreak havoc when dispensed...
Cheers!