It was kind of vague to me:

The action merging QoL feature, is that just for shifting actions we have already (e.g. Earthly Star) or that's gonna be a feature for the 1-2-3 filler combos that don't have the need to branch?

I thought the former, but then I saw Pictomancer play and they have a cast 1-2-3 combo using the same button.