And no, GDPR is from my own experience not easy to understand or apply in practice - except for PPT toy problems.
There is a lot of more behind the articles in GDPR since it is partly based on legal instruments on data protection from 1981(!), “Convention 108” and EU legislation from 1995. Case law, opinions from data protection authorities etc. are important.
In general though, I would not be worried about big fines or anything as long as you try to follow GDPR and you are not doing things the data subjects does not want you to do with their data.
In general, the best GDPR compliance test is how it feels in your gut, after learning the principles.
At the point in time you process personal data against the interests of the data subject (for example building marketing profiles, sharing data with third parties w/o request from the data subject, etc) it is time to start worrying and make sure you cross the i’s... GDPR is pretty permissive in general IMO but make sure you know the details.