Parsing nested JSON into R List

I have a pretty straight forward JSON object that I am trying to parse into a list of objects for downstream processing and use. The JSON structure is dynamic but here is an example of the structure I am trying to handle

json_string <- '{
  "instances": [
    {
      "feature_1": [
        { "col1":  "value1", "col2": "value2" },
        { "col1":  "value2", "col2":  "value3" },
        { "col1":  "value4", "col2":  "value5" }
      ],
      "feature_2": "raw_value",
      "feature_3": [ "list_value_1", "list_value_2"],
      "feature_4": {
        "feature_4_prop1": "prop_value_1",
        "feature_4_prop2": "prop_value_2"
      }
    },
    {
      "feature_1": [
        { "col1":  "value1", "col2": "value2" },
        { "col1":  "value2", "col2":  "value3" },
        { "col1":  "value4", "col2":  "value5" }
      ],
      "feature_2": "raw_value",
      "feature_3": [ "only_1_value"],
      "feature_4": {
        "feature_4_prop1": "prop_value_1",
        "feature_4_prop2": "prop_value_2"
      }
    }
  ]
}'

library(jsonlite)
parsed_data <- jsonlite::fromJSON(json_string, simplifyDataFrame=TRUE, flatten=FALSE)
row <- parsed_data$instances[1,]

print("fin")

I want to be able to parse this and then obtain the instances inside of a list (or data.frame) i.e parsed_data$instances

When I view a single row / item of this list via row <- parsed_data$instances[1,] I would like it to assume the following structure for each column.

feature_1 is a data.table with the columns of col_1, col2.
feature_2 is a raw value as indicated.
feature_3 to be a list even if a single value
feature_4 to be an object so that row$feature_4 allows me to access the properties.

I have tried to do this via jsonlite:fromJSON(json_data, simplifyDataFrame=TRUE, flatten=FALSE) where flatten was set to false in case only 1 instance is set.

The issue is when I inspect a row or assign a row I see that feature_1 is given to me as a list and in order to access the table itself I would have to do something like the following:

row <- parsed_data$instances[1,]
feature_1_table <- row[['feature_1']][[1]]

Is there a way that I can parse a JSON structure above to achieve my desired results?

edit: Including Screenshot of debugger to show that jsonlite is not doing this out of the box and that feature_1 is showing as a property that is list of length 1.