HTTP POST response - http

Question

I have a little problem. I wrote a program on ESP32 which records some data and save it to SD card then data is read line by line and send to app which put it in postgres database. The problem is, sometimes data is being write to database long and sometimes it took like 0.5 sec, so i get response after 10 sec, sometimes after 5s, etc. Here is problem, on ESP I cant figure out how to wait to get response, my program just give me error but the data is sucessfully sent, it shows in database. So here is my question, how am I supposed to wait for response and then I can run rest of code.
And second question with the same program. As Im using sd card adapter and writing data to files, after i get response = 200 from server I want to clear files. What I did is I just delete both files (Im saving data to 2 files) and when i need to write data again to those files I create them. I suppose it not the most optimal way of doing it but couldnt find any method to clear files content or I just missed it somehow. Any suggestion on how I could do it another way?
if (WiFi.status() == WL_CONNECTED && flag == false){
createFile();
HTTPClient http;
http.begin("http://192.168.xx.xxx/xxxxxxx");
http.addHeader("Content-Type", "application/json");
String h_str = String(h);
if (h < 10)
{
h_str = "0" + String(h);
}
String m_str = String(m);
if (m < 10)
{
m_str = "0" + String(m);
}
String s_str = String(s);
if (s < 10)
{
s_str = "0" + String(s);
}
String totalTime_HMS = h_str + ":" + m_str + ":" + s_str;
//Srial.println(totalTime_HMS)
doc["name"] = "Machine_1";
doc["totalTime_HMS"] = totalTime_HMS;
readLinesSDON(SD, "/dataON.txt");
delay(50);
readLinesSDOFF(SD, "/dataOFF.txt");
String requestBody;
serializeJson(doc, requestBody);
int httpResponseCode = http.POST(requestBody);
flag = true;
if (httpResponseCode > 0)
{
String response = http.getString();
Serial.println(httpResponseCode);
Serial.println(response);
deleteFile(SD, "/dataON.txt");
deleteFile(SD, "/dataOFF.txt");
//clearing temp arr - debug only
for (int i = 0; i < numL; i++)
{
timesON[i] = "0";
timesOFF[i] = "0";
}
}
else
{
Serial.printf("Error occurred while sending HTTP POST");
}
}