Getting the Kingdom title is really easy from there. Then get the ambition to become king and get them under your control. Then just wait for the revolt in the South and help out your fellow Polish regents. Or be careful that they don't rebel later, otherwise Denmark tends to swoop in for a quick conquest.

If you have still enough room for another county, get the primary one from them.

Immediately subjugate Pomerania in the North. You get three counts and a duke title with no vassals. Great Poland makes reforming the Slavic faith fairly easy.