You can’t do what you want with a castsequence because all spells in the sequence need to follow the same logic so the help/harm portions will make the spells conflict with one another.
You’re better off using a help/harm macro and mousing over the heal target.
The modifer portion in these macros is just so you can alt-cast to force cast the heal on yourself.
Alternatively, you can use this macro for your damage spell and just keep the tank or another ally targeted the whole time.