The thing is, a 1-2-3 combo is something that's been talked about quite a lot as a specific example of what many DPS advocates are not asking for. On its own, a 1-2-3 combo is technically giving you different buttons to press, and it does add more visual disparity between your general gameplay, but if it exclusively exists to be a 1-2-3 combo--that is to say, offers no branching or any sort of resource earned from completing the combo, that it's not really creating any different of an experience than what we have with single button spam. It also doesn't really resolve the main aspect of this thread topic which is reducing the weight of filler-type actions.

One thing you could argue for tanks is that the 1-2-3 combo does act as a buffer for their self sustain. Warrior needs to wait till it reaches Storm's Path in order to recover HP, and it has the alternative Storm's Eye to maintain its main damage buff. Additionally, it also regulates the natural generation of their Beast Gauge. The same is basically true for Dark Knight as well, adding in that it also gets MP generation during Syphon Strike. If we look at the one example of a caster combo in the game, we have Red Mage. It's mechanically not a combo, but functionally it is a 1-2 combo. But then you also have Verstone and Verfire procs, as well as Acceleration and Swiftcast which are important parts of combo management. Meaning Red Mage is very often making choices between a wide number of possible 2-part combos:

1-2, 1-3, 2-2, 2-3, 3-2, 3-3, 4-2, 4-3, 5-2, 5-3 are all different variations of Red Mage that you can pick from. If we were going to go down the route of having a caster combo on a healer, I think it'd make more sense to go down a route like that rather than taking the MCH angle.

For example, rather than just having Dosis A, Dosis B, and Dosis C (for lack of appropriate names) why not do something like...

Dosis, Biodosis, and Pathodosis (temp names). Instead of generating Addersgall naturally over time, you instead have 2 meters, one for Addersgall and one for Addersting. Dosis > Biodosis makes progress toward your Addersgall while Dosis > Pathodosis makes progress toward your Addersting. In that case, Toxikon as a mobility spell makes more sense as a resource (assuming your normal combo spells have have cast times).

Or another angle would be to say Dosis has no Kardia effect while Biodosis restores HP when comboed from Dosis, while Pathodosis grants your Kardia target a mitigation when comboed from Dosis. In that case it functions similarly to how tank combos work, only for your Kardia target instead of always for you.