This is more of a future thing right now (January 2024), but it looks like there are some cocoa-free chocolate substitutes being developed that might work for this, for example one that's based on sunflower seeds and oats processed in a similar manner to cocoa (fermenting and roasting), or one that's based on barley and carob. Specifically these two are only selling B2B in a limited area right now, though the sunflower stuff (ChoViva) sounds like they might plan to sell directly at some point.
Both of them claim you can use their product like regular chocolate, which sounds like using it for chocolate sauce would be possible. Though I do want to note at least the sunflower/oat stuff seems to be mainly used as an ingredient in cereals, "chocolate" chip cookies and the like, where any chocolate taste might well just be placebo - you expect chocolate, you taste chocolate.
At least for the former, they apparently developed their product using a fancy food processor and some kitchen-scale fermenters, so if you're really dedicated, you might be able to reverse-engineer something similar.