Can Search and Replace be used to Append to Json files with RipGrep and Sed?

It’s not really append what I’m looking for but because I had future noobs in mind, I took the liberty of including it in the title.

I am looking to search for/capture a line-ending " and a ] in the next line (end of file) in json files (many of them). I am using multiline option in ripgrep and forwarding the list of results to be replaced with sed. Running a sole rg without -l lists my results without problems. The problem is with the sed part.

cd path/to/folder
rg -l --multiline '(")(n])' regex/to/path/to/folders/settings.json | xargs sed -i 's/(")(n])/1,nInputStringn2/' *.json

• I am searching recursively in multiple folders (the regex is correct as I used it for other replacements including git).

What I want is to put a comma after the comma-less line (with the captured quote at the end), put in a new line, put in a string (I’ll handle the spaces and quotes myself), another new line and a ] to close the json settings files.

Sample:

[
  "data1",
  "data2",
  "data(n)"
]

Desired result:

[
  "data1",
  "data2",
  "data(n)",
  "inputstring"
]

No matter how I tried, I couldn’t make it work. The *.json at the end is probably superfluous, just as the brackets in the rg command part. But these are not the problem. What is the problem? Am I missing a sed option/flag? Is there some character that needs to be escaped? Maybe the syntax with the groups in the replacements?