Over the past year or so, I've noticed that when trying to pull darker roasted coffees like Counter Culture's Forty-Six or even Big Trouble to some extent I get an espresso that's dominated by mostly ash and a hint of chocolate/caramel. When this kind of shot is combined with milk, it's mostly ashy with a little bit of sweetness from the milk.. it's definitely not the most pleasant thing. I was wondering what some of you might recommend to try to maximize the caramel and chocolate notes? I've tried pulling a tighter ratio, like 1:1.5, and even a 1:2.5. My dose is locked in at 17 grams for now. I'd really appreciate some suggestions, I used to really dig these comfort-like blends, but now I'm just having a hard time getting past the dominating roasty characteristics.

Typically I've had much better success with the type of roast levels you describe by pulling at a lower brew water temperature and a declining pressure profile.

Pressure profiling isn't really possible for me, but I have tried a series of different flush routines.

Can you adjust your p-stat to lower the boiler pressure to effectively lower the brew water temp? If not then you might need to resign yourself to matching the type of roasts/beans that play well with your machine, which isn't that unusual BTW.

Do you have the option to switch baskets? If possible you can go for a slower flowing basket, like a typical Faema one, which will allow you to grind coarser at the same dose. Barring that I agree that trying to get your brew temp as low as possible would be the best thing if you can.
You stated that pressure profiling isn't an option, but can you change your overall pump pressure? You might try pulling at 6 or 7 bar and see how that goes.

I don't yet have the ability to reduce the overall pressure (haven't gotten around to retrofitting an OPV yet). I do have an Espresso Parts HQ-14g, but I've been using strictly the 18g VST for this coffee.. would you advise trying that out?

I would suggest you don't use VST for dark roasts. They force you to grind way too fine. I use VST for the grand majority of my coffees but I won't use them for dark roasts because they always taste overextracted out of them. Although with the 14g HQ, I'm not sure how permissive it will be to a 17g dose. I suppose you could give it a shot.

ridgeless hq-14g is about the same size as the 18g vst, it does have far fewer holes which may permit a coarser grind.

I tried this bag of coffee from collectivo without reading it and it was dark. Fixed a majority of the ash with a lower temp (200F -> 195F) and using the stock basket.

Remember our interchange in another thread about the cooling flush, Luke? I bet your temp's too high because you're only flushing a few ounces. I flush at least two 8 oz cups after several minutes of idling, but if there are still bubbles in it I do a third cup & they're gone. Be ready to lock the PF and push that button as soon as the brew temp is down. I flush into a clear glass measuring cup so I can see the water clearly. If it looks like club soda, it's still too hot.