Healer balance is very, very good right now.
AST + SCH has a slight lead, but AST + WHM is so close it's very, very viable.
Now, you can say the gameplay/'fun' factor is lacking and I can agree, and some QoL changes could help.
But the balance is not really a problem. AST is the strongest healer by far so you could argue for a bit of a nerf, but right now all 3 healers can be used pretty equally as long as one of them is AST. Granted it's not ideal to have one healer be required, but it's about time the required healer isn't SCH for a change, and as long as all of them are desirable - I call that pretty good.
Unfortunately you can't buff WHM or SCH because if SE did that, one of the top two combos would become drastically better and become the 'required' raid composition. There's room for QoL changes, but nothing that would directly increase their potencies, etc.