I work via Upwork as a VA too, but if I ever needed to hire another VA for something, I will then be someone too busy to read such long proposals.
Also, clients mostly do not pay much attention to experience of their candidates. They are mainly looking for someone who can describe themselves as a good fit. Thus, you need to read proposals carefully and look up the client, his history on Upwork. You use this data to craft a killer proposal.
There is no ideal length for proposals. As you probably noticed, many proposals have additional questions. If you spot these, have only one short para in the main cover letter, and concentrate fully t these additional questions.
If there is just the Cover Letter section, just imagine you're the client in need of that and think about what is the thing they need to be done and how. Nothing more, nothing less.
Also, increase your default hourly rate to at least $15.
Your profile Overview is well organized, so should be your cover letters. But one thing - your English test result is very high, but the Profile Overview does not show that. As a native Serbian speaker I clearly recognize Serbian word order and Serbian phrases, and all that contradicts with the test result. People who do speak English well, both native and non-native may reckon that you perhaps cheated on the English test.
Anyway, best wishes.
Keep trying, 20 proposals is NOTHING, I've been working on UpWork for years and have great track record and it takes me min 30-40 proposals to get a couple of inteviews and double that to get hired. It's a number's game, the more you send, the more chances you'll get to get hired.
Reduce your initial proposal to just a couple of sentenses, but make them GREAT. Always keep in mind the competition, you've got at least 50 other people who'll submit a proposal.