I do know darker roasted coffees offer more crema because of the extra CO2 gases being released. However dark roasted just are not something my palette enjoys and can taste pretty murky, and given the current trends in coffee and consumers being more educated, I would stay away from dark roasts.
I think your best bet for enhancing crema is using fresh roasted coffee and really dialing in your espresso. I’m not sure what kind of equipment your working on, but espresso usually tastes it’s best at a 1:1.5 ratio or 1:2 ratio. So if your putting in 18g of roasted coffee you want to extract anywhere from 27g-36g yielded output, in probably 25-35 second time frame for starters. Play around and see what works for you.
Do know that if you want crema for latte art that’s fine, however crema is the most bitter part of your shot given that it is composed of mostly bean fiber and is essentially an emulsification of the coffees oils and CO2 being released from the roasting process. The oils in the coffee repel the water but under high pressure, aka 9bars of atmospheric pressure bond and form the foam like crema. There a few really good articles out there on Perfect Daily Grind and Coffee Geek about the emulsification of crema. I definitely suggest reading up on that if your interested.
Hope this is insightful!
All the best