Having a hard time editting a file in C

I am trying to write some code that is supposed to add kernel parameters in the file /etc/default/grub inside this line:

GRUB_CMDLINE_LINUX_DEFAULT="quiet pci=noaer resume=UUID=57661f93-9206-48ba-95b4-68fd86519872 loglevel=3 audit=0 amdgpu.ppfeaturemask=0xffffffff"

The kernel parameters is supposed to go inside the quotes (""s). My attempt at doing it was to replace the last " with

the kernel parameter" \n\0

However, that is not working well for me and I get all sorts of formatting issues with my code. I am very new to C so I might not know how to do simple things in C. Here is my code below:

void add_grub_param(char param[100]){
    // Code to add a grub_parameter
    FILE * grub_config = fopen("/etc/default/grub","r");
    FILE * grub_temp = fopen("grub-temp","w");

    char singleLine [5000];
    while(fgets(singleLine, 5000, grub_config)){
        if (strstr(singleLine, "GRUB_CMDLINE_LINUX_DEFAULT=") != NULL){
            if (!strstr(singleLine, param)){
                // I can use a boolean that gets trigerred in the first occurance of "
                // In the second appearance of " , we can replace it with the text
                char newLine[6000];
                
                char textToAdd[150];
                strcpy(textToAdd, " ");
                strcat(textToAdd, param);
                strcat(textToAdd, "\"");
                printf(textToAdd);
                int counter = 0;
                bool first_occurence = false;
                while (true){
                    if (singleLine[counter] == '\"'){
                        if (!first_occurence){
                            first_occurence = true;
                        } else{
                            strcat(newLine, textToAdd);
                            fputs(newLine, grub_temp);
                            break;
                        }
                    }
                    newLine[counter] = singleLine[counter];
                    counter ++;
                }
            } else {
                fputs(singleLine, grub_temp);
            }
        }
        else
            {
                fputs(singleLine, grub_temp);
        }
    }
    fclose(grub_config);
    fclose(grub_temp);
    system("sudo mv grub-temp /etc/default/grub");
    printf("GRUB config file successfully generated. \n");
}

Any insight on the problem would be greatly appreciated. Thank you.