Honestly if you are nervous about crowds you perhaps should not be travelling. Crowds are going to a fact of life right now when travelling. I haven't been to Greece yet but in the US the airports have been slammed this summer just awful and worse than normal because the experience business travelers are gone and it is mostly families who are less familiar with the whole process.
The best you can do is protect yourself. Be vaccinated, wear N95 masks (wear them correctly--read instructions) and do your best to maintain what distance you can.