I don't have time to do all the documentation, test cases, strategy,
test plan, etc.
I almost always recommend producing the minimum necessary documentation. Call it lean documentation, minimum viable documentation, whatever. You want just enough that helps plan and guide the testing you need to do. Anything more starts to lose value very quickly.
Typically this means defining a strategy (high level test ideas) and setting up whatever systems, data, structures I need before I get into my testing. Test plan documents usually aren't necessary but you shouldn't spend much time on them anyways. Google is famous for their 10-minute test plans. Most test plans are throw away documents - once they are created, they are almost never used again.
I don't know what you mean when you say test cases. Hopefully you don't mean heavily scripted / written tests that say "step 1 do this, with this expected result". I've found this to be a waste of time and just not valuable (most times).
Some tests will need to be designed in advance. Some may even need great detail. Most won't.
Currently I am the only one responsible for the quality of the
You're saying there is no one in the company other than yourself who is responsible for the quality of the products? The programmers you work with aren't responsible for building quality in? There are no managers, executives or a CEO that takes responsibility for the companies product? If this is truly the case there is nothing you can do. After all you can't test quality in.
Final words of advice:
If you are worried about your value (that's a good thing), be smart with your time. If you feel like you're spending too much time on a test or effort that isn't producing valuable results, stop and re-evaluate.