So according to my link you should count 50% of your time spent in Poland studying, when applying for " pobyt rezydenta długoterminowego"
The period of stay referred to above includes half of the period of stay of a foreigner on the basis of a visa granted in connection with studies or vocational training or on the basis of a residence permit for a fixed period, granted in connection with the start or continuation of full-time or part-time studies PhD.
and you write:
I have finished 4 academic-years of music degree in Warsaw - 1 year of Staż artystyczny (it's meant to be a post-graduate diploma course but in my case it was the starter) and 3-year-long Bachelor of Music course.
I am not sure what is " - 1 year of Staż artystyczny", but I guess that it's 4 years study + 1 work + 3 study is converted to (4 + 3)/2 + 1 = 4.5. So if you can stay up to full 5 years and meet all other conditions, like:
- you haven't left Poland in last 5 years for more than 6 months per year and less than 10 months total
- you have a job
- you have insurance
, then you can apply for long-term residence permit to your "wojewoda"
So now if you don't have work visa you should find some company which would like to sponsor your visa. Unfortunately as far as I know artists don't have it easy when searching for job.
But I think that graduates of Polish universities have a right for 1 or 2 year work permit. So IMHO you should go into this direction. Then learn what does it mean "stable resources sufficient to live without recourse to the social assistance system of the EU country concerned;" and find a job which meets this criteria.
If it's impossible and you have some money you can fake employment. For example you officially work for somebody for 1 year, you get average salary, you pay all taxes, but in realty you return half (or more) of your net income to employer.
Here I read about graduated of Polish universities not requiring work permit:
At the same time, in accordance with the Regulation of the Minister of Labor and Social Policy of 20 July 2011 on cases in which it is permissible to perform work for a foreigner on the territory of the Republic of Poland without the need to obtain a work permit (Journal of Laws of 2011, No. 155 , item 919) from the obligation to obtain a work permit, foreigners were released:(...) graduates of Polish upper secondary schools, full-time studies or full-time doctoral studies at Polish universities, at scientific institutes of the Polish Academy of Sciences or research institutes operating on the basis of regulations on research institutes;
This website explains also procedures, so you should search how to proceed with work search/permit there IMHO.