also luggage question--airline booking uncertainties

If just switching planes in Toronto or Montreal there would be no immigration or customs. If planning to leave the airport, then that is a different issue. If you don't have preselected seats you are on your own for a bunch of middle seats. Aegean air is fine or at least it was a couple of years ago. Reminded me of SW airline.

We took Air Aegean just a few weeks ago from London to Athens , then next day took Olympic air to Paros . No complaints - we flew ec because to us it’s a short flight ( 4 hrs I think ) and we’re even given a meal that was fine .
I took my purse and a small
Carryon onboard and paid for a checked bag of 23 kg which I think is generous .

We then till ferries to various islands and eventually after three weeks returned by ferry to Athens where we then took Air Transavia to Paris , which was also fine .

We flew Aegean on a connecting flight in April and it was fine in economy and it was on time into Athens. We too were served a complimentary simple and actually tasty lunch. Would not hesitate to fly with them again.

Aegean Air has very high ratings.

I've flow from Athens to several islands with Aegean and had no issues.

Everything went smoothly, on time and safely.

Can't ask for anything more than that.

Just to remind you that Aegean as well as Air Canada and United are Star Alliance members.
If you have a journey in several legs with Aegean and one of these airlines you can book it in one ticket

Aegean is regularly ranked among the best European companies. I have flown multiple times with Aegean ( and Olympic, its domestic subsidiary), last time was last month, never had any problems.

I just ran an itinerary on Google flights from SFO to ATH for a 09/13/2023 departure and a 09/27/2023. Nearly all of the routes are one stop with the majority of the intermediate stops being in various European cities. You don't necessarily need to go through Toronto or Montreal. Also, when you book the majority of the airlines will let you select your seats. If you will give us your travel dates someone on these boards will be happy to help.

Seats--if you bought the cheapest tickets that don't include pre-selecting seats, then you may not sit together. They will try to put you together but if the plane if full and others have pre-selected seats because they paid more, nothing is guaranteed.

Each airline has their own carry on rules. You have to follow the carry on rules for whomever is flying the plane. You might get a gate agent who is sympathetic or you might not.

hard to recommend flights, when people don't share what their priorities are ... is it (1) the shortest total time of travel to reach Greece (2) assuring arrival in the AM, with 99% assurance of on-time arrival (3) saving $$ by a transfer in Europe, w. afternoon-evening arrival, plus greater chance of delay or (4) any arrival time or trip lengh, in order to "fly free" through use of points.

The shortest & promptest flights (and alas few bargains) are the nonstrops from N America -- Air Canada, Delta, AA, United, ?UAR? -- that leave late in day from East Coast, arrive ATH 9 - 11 am. Ezperiences w EU transfers vary, the one precaution most Greece-travel vets agree on... avoid Chas DeG airport at all costs.

As for luggage, as others note, when changing to Aegean in major European airports, there won't be a problem w carryon because all these planes are larger... but they may not go to islands you want. You run into the nO carrry-on problem when going from ATH on domestic flight to Naxos, Milos, a couple other short-runway islands that only use smaller (modern) turbo-prop planes w limited cabin storage.