Sed - Help Matching with Multi-Line Match

I am fairly new to regexes and it appears I am missing something in my understanding. I have the following file, and I am trying to insert a value if it is missing. The file looks like

[composefs]
enabled = yes
[sysroot]
readonly = true

I would like to insert the following only if it is absent

[etc]
transient = true

My sed search and replace looks like sed -zi '/^[etc]\n^transient = true/!s/$/[etc]\ntransient = true/' file When I run sed it adds the value as expected, but it keeps adding the value un subsequent runs so I assume my matching is not working. How can I do this correctly?