You don't need ATPL theory, but it is more efficient than that for the CPL/IR in the long run.
It is not impossible, but has to be approached in the right frame of mind, and 40 hours in the classroom.
If you have an ICAO CPL (you do), you only need 400 hours of study anyway.
And just to point out - what may seem irrelevant at this stage will not be irrelevant later in your career, as many people found out when their airline's computers went down and the paperwork had to be done manually - back to basics!
Having said that, some questions are a little strange to say the least, so you will need guidance through that minefield.
To answer your question, you need a ground school provider and a flight trainer. Expect the process to take about 6 months.
Good luck!