Athens size and navigation, customs, etc

I haven't been in the Canadian airports you mentioned, but ATH is relatively small by European or US standards. Much smaller and simpler than Vancouver. Straightforward layout, bilingual signage, one terminal.

Customs will be a simple matter of walking through a door labeled "Nothing to Declare." Immigration, aka passport control, will happen at the airport where you first enter the Schengen area, probably in western Europe unless you have a transatlantic flight straight to ATH or a connection through London. How long it takes will depend on how many international flights are arriving at about the same time and what staffing levels are on a particular day and time. Past experience doesn't necessarily foretell future results. ;-)

I have been to all the airports you mentioned except Calgary and Athens is much easier and smaller--especially than Toronto!

We were there last summer (2022) as well as in 2018 and it did not seem much different than earlier.

These are excellent thoughts. We fly from Edmonton through Toronto and then directly to Athens. So yes, Athens will be the first Schengen Area (if I'm processing clearly, ha!).

Athens airport is not huge and has only one building with two levels.
Arrivals are on the lower level, departures on the upper level.
All signs are in Greek and English.

I found this video so that you can have an idea of ​​the airport when you arrive from a non-Schengen area.

https://www.youtube.com/watch?v=VV_40zSrKpI

You can skip the first 1.30 minutes, it's the plane exit.

Once outside, at 5.33 the guy goes in the wrong direction to take the metro. After a few irrelevant comments he finds the right way at 7.06.

Talking, filming and orientation are not compatible! :)))

Then it's the metro ride to Syntagma Square.

Will you be connecting to an onward flight or staying in Athens on day of arrival?
In any event you have had good advice so far.
There are several transatlantic flights all arriving within an hour of each other around noonish. If you have carry on bags only you can scoot to passport control as soon as you get off the aircraft which can be helpful to beat the crowds. If you have checked luggage you have to get to the luggage carousel which will add an additional 12 to 18 minutes until you can go to Passport control.

In our experience Passport control can take 10 to 30 minutes depending on the crowds. Walk to arrivals exit door and walk through and you will be in the terminal. Athens airport is modern, well laid out and easy to navigate.

If you have a connecting flight you can book your boarding pass with Sky Express or Aegean 48 hours before your transatlantic flight
So you are already checked in before you leave Canada.

If you are connecting you should probably have 2 hours to make the connection.
Procedure is, go up one floor and you are in departures.
If you have to check in bags there is a machine where you can print luggage tags and put them on a belt. If you have more time you can also just go to the proper airline counter to deposit your bags.

Next step is to go through security. Athens airport is very efficient and security is generally quite fast. After security you are in the departure area. In our experience you will have sufficient time to stop for a quick snack and a beer before you need to be in the departure gate.

Do keep in mind that connecting flights require a bus ride from the departure gate out to the aircraft so be sure to be in the departure gate area well before flight time.

One nice thing about the AC flight arrival time is it arrives mid day and the early morning rush of flights have already departed so there are no big crowds.