Gatling - how to run gatling tests to load test same rest URI request with different query param and the corresponding request payload - gatling

Question

I do have a test scenario to load test the API request with a different employee id (query param) and the corresponding payload, i do have all URIs handy to test, and Each POST request should execute only once.
so i assume, scn.inject(atOnceUser(1)) should be just 1 in my case, as user is a just single execution of a scenario from start to finish. Can someone confirm if my understanding is right. here i want all three POST requests to execute only once simultaneously.
I am planning to use scala3 to test the same. (like 100's of POST request simultaneously, with a different employee id each time, and submit POST request only once for each employee id)
POST: http://localhost:8080/orders/00000/product/item?employeeId=1234
payload A
POST: http://localhost:8080/orders/00000/product/item?employeeId=5678
payload B
POST: http://localhost:8080/orders/00000/product/item?employeeId=8352
payload C
and once the first round of testing is done, now i would like to execute the same POST request URIs (100s of POST URI's like above) with the different payload in round 2. In Round 2, the URI is still going to be the same with the same query param (employee id), but payload will be different from what i use for the previous round 1.
and again, i would like to run all below requests only once and run each request simultaneously. ((like 100's of POST request simultaneously)
POST: http://localhost:8080/orders/00000/product/item?employeeId=1234
payload D
POST: http://localhost:8080/orders/00000/product/item?employeeId=5678
payload E
POST: http://localhost:8080/orders/00000/product/item?employeeId=8352
payload F
and continue the same process as above for Round 3 and Round 4, with the same URI but with different payload, again execute all POST requests simultaneously, but execute each request only once.
Can you please help me with the approach to get this working?
Can you please advise how i should plan to store all these 100+ payload in the feeder file for each round, should i be having a single feeder file with 100+ post request body and Can you please advise if i should be storing all 100+ URIs with the query param in the feeder file as well? Note: for each round , the payload will be different
ffor eg: Round 1- 100+ POST URIs --> and corresponding 100+ request body
Round 2 - same 100+ POST URIs as Round 1 --> but this time different 100+ request body (corresponding payload for each URI)
Round 3 - same 100+ POST URIs as Round 1 --> but this time again different 100+ request body (corresponding payload for each URI)
Each URI has its own request payload for each round.