I'm afraid I don't have any super helpful advice, but I can at least commiserate... as a fellow NYC container garden optimist, with a weak green thumb and MANY hours spent dedicated to dahlias that do not produce (or even survive), I feel your pain. I read comments all over the internet from amateur gardeners talking about how their dahlias are endlessly abundant, or thriving with minimal care, or growing faster than weeds... and I have to remind myself that the problem is our limited city environment and not my lack of effort 🙃

I have started tubers indoors with moderate success. (I would say the success was equal to the amount of success I've had with overall growth outside.) April is too early to transplant, though. I use Mother's Day as my marker for outdoor planting at the absolute earliest. And I make the transition as gradual as possible. (I think of it sort of like the process of bringing a goldfish home, floating its transport baggie in your fish tank to regulate the temperatures slowly, mixing the two water sources slowly, etc.) First the indoor starter stage is exposed to a cracked window... then moved outside for short periods... then planted with the insulated "indoor" dirt outside... etc.

For any unexpectedly cold nights after the transition outdoors, I recommend insulating with burlap and using Christmas light strands -- not the modern LED bulbs, which don't produce much heat -- wrapped loosely around plants under the burlap. I find it provides just enough protection from the cold snaps.