isn't it true, that flying a glider with L/D 50 and sink rate of 0,5 m/s creates much smaller risk of outlanding altogether ?

Smaller, but probably not much smaller. And to become the pilot you want to be, you'll need to get comfortable with outlandings.


there are lots of large agricultural fields in that part of Poland.

Yes, much of Poland is very glider-friendly (when crops aren't tall). But after outlanding you have the challenge of de-rigging the Nimbus. For many pilots, that can lead to "Sure, it's a nice day: probably 300+ km. But just a little uncertain. Best not to fly away from home and risk the chance of outlanding."