I am 16 to 17 years older than you. I have traveled solo to all three countries. Each trip was 13 nights on the ground, one night on the plane on the way there. My only experience with Islands in Greece was, I went to Crete for 2 nights and most of 3 days. I would have been as happy if I had gone to Olympia in Western Greece. There was nothing blatantly wrong with my itinerary. I just didn't give myself what most opinions would say is a good amount of time on Crete. I went to Nafplio; Heraklion and Chania on Crete; Delphi and Athens. I don't see why Greece is a place to only go with friends. My solo trip worked ok. Crete has enough to occupy you for 3 to 6 days. Rhodes has a neighborhood that was a Jewish quarter, and a synagogue built in 1577. If I go back to Greece, I would probably include the old town in the city of Rhodes, possibly Olympia, and towns and ruins I didn't see in my previous trip to Greece. So far in traveling I have been going to different places each time and not back to the same places. Go to Greece because Classical era Ancient Greece seems like a grandiose and influential civilization. Crete and Rhodes have plenty to do. Mainland Greece has plenty of museums and old ruins. Greece has so much to do.
In Italy I didn't make time for Venice. I went to Florence, day trip to Pisa, Pompeii and a small part of Naples, and Rome.
In Spain I skipped Barcelona. I went to Seville, Granada, Cordoba, and Madrid and a day trip to Toledo.
I couldn't discern that Spain people were more (or less) friendly that Italians or Greeks.
Chicago people were definitely unfriendly and the opposite of helpful. You can ask for details.
I went to Italy, then Greece, then Spain. Actually I went to Spain in March 2022. Starting when I went to Italy, I wrote myself a supposed day-by-day itinerary for that trip and each subsequent trip. I made my reservations 3 to 6 months or more in advance. I don't know whether this was necessary or whether I just needed something to occupy myself with. 2 weeks is too close to the last minute to properly plan a trip. You need advance tickets for many of the most popular sights.
I have stayed in 11 hostels in addition to low-end hotels. I was 32, at least ten years older than you, when I first stayed in a hostel. The crappiest hostel I stayed in was actually in Chicago (Illinois, USA). I had trouble sleeping due to some guy snoring, and there was no lockers. Every other hostel had lockers.
In Spain I stayed in 4 hotels. I did not stay in any hostels.