Let's take the example, that the 123 combo has cast times of instant, 1s, 2s for it's steps. You can spam 111111 for instantcasts over and over, for mobility. Beyond that, the rest is up to you:
Can you explain how you're going to incentivize the player to do their 123 'properly', without it causing a conflict with your previously stated stance of 'if there's too much complexity, the healer won't be able to deal damage and it'll lead to enrages'?
edit:
SE has a design they're running with (2min meta) and every time a suggestion was ever made to break that 2min meta (eg, me suggesting a 18s DOT), you, YOU yourself, said that it would not work because 120 does not divide cleanly by 18. That the DOT would not line up with every 2min window and that's bad. So, not our problem that SE's decided on everything being centered around the 120s mark, we as random forum users can only play into the 2min meta with our designs
Beyond that, you appear to be saying with the last line that, for example, giving SCH a total of 3 DOTs, and giving WHM what I've suggested for it, are 'the same' because both have at least one DOT (Dia/Bio for example). How is the gameplay of WHM as I've described it, the same as SCH with 3 dots of various durations, the same as AST with 'Minor Arcana is the main focus of the class now'? We do agree that the healers should not all have the same damage rotation. But that doesn't mean they can't use similar 'design facets' as one another, provided they are done in different enough ways. For example, while some of the healers have a DOT, SCH has several DOTs. While WHM has a 15s CD you don't think about too much (Banish), AST would have a 15s you absolutely would want to think about (Minor Arcana card effects). The fact that the two share a CD does not 'make them the same', else we could say Assize is the same as Life Surge on DRG 'cos they're both 40s CDs'.
If you were to try and make 4 different healers, that have absolutely zero overlap in their designs (eg if one has even a single DOT, none of the other 3 can have a DOT), you'd run out of design space unbelievably fast. It'd also mean that, since one healer has 'Glare, the filler spell', the other three healers cannot have a filler spell and must have some other way of 'filling' while their CDs come back