API Testing - Best Strategy to adopt

I have few questions for you. Let me give some details before I throw the questions.

Recently I have been involved in Rest api test automation. We have a single rest api to fetch resources,( this api is basically used in our web app UI for different business workflows to get data) though its the same resource api, the actual endpoint varies accordingly for different situations .i.e the query parameters passed in the api url gets differed based on the business workflow.

For example something like

`./getListOfUsers?criteria=agedLessThanTen ../getListOfUsers?criteria=agedBetweenTenAndTwenty ../getListOfUsers?criteria=agedBetweenTwentyAndThirty etc

As there is only one api and as the business workflow do not demand it, we don't have chaining requests between apis So the test is just hitting on individual endpoints and validating the response content. The response are validated based on a supplied test data. So the test data file has list of users expecting when hit on each particular endpoint. i.e the test file is like a static content which will be used to check the response content everytime we hit on it... if the actual response retreived from server deviating with our supplied testdata,it will be a fialure. (also tests are there for no content respose,with out auth etc)

This test is ok to confirm the endpoints are working and response content is good or not .

My actual questions are on the test strategy or business covergae here, 1)Is such single hit on the api end point is sufficient here or not.. 2)or same endpoint should be hit again for other scenarios or not, especially when the above given example of endpoints actually complimenting each other and a regression issues that might happen, can possible captured in anyof it ? 3)If api endpoints are complimenting each other, adding more tests , will it be just duplicate of tests/ more maintainance / and other problems later on and should we avoid it ? if its not giving values ? 4)Whats the general trend on API automation regarding the covergae? . I beleive it should be utilzed to test the business integration flows, and scenarios if it demands but for situations like this , is it really required 5) also should we keep this point in mind here ?, that automation is not to replace manual testing, but only to compliment it . and attempt to automate every possible scenario is not going to give value and only will give maintaince trouble later ?

Thanks