Safe to use metro Athens airport to Monasteraki around 10 pm?

Arriving in Athens center around 11:00 p.m., you will still have a lot of people in all the bars and restaurants, and Monastiraki square will be very lively and noisy, especially since it will be a Friday evening.

So I wouldn't worry about the "night" part as if you were arriving in a dead, deserted town. The most difficult part could be to go from Monastiraki square to your hotel via crowded sidewalks.

The metro is safe and very easy to use. For Monastiraki the journey takes 40 minutes and there is a metro departure every 7 minutes.

But the Airport-Monastiraki route taken by disoriented and tired tourists arriving in Athens is also a favorite playground for pickpockets, so just take the usual precautions by watching your bags and pockets (keep your backpack in front of you) and your environment, especially when you arrive at Monastiraki station and get out of the train, as well as on the stairs or escalators.

If you are still apprehensive about taking the metro or walking to your hotel, take a taxi.
It's 40€ flat rate until midnight.(55€ after midnight)
The metro ticket to Athens cost 9€.

Thank you! Yes, the only time in my life I've ever been pickpocketed was on the metro in Chicago from the airport into town...because i had let my guard down, and had all my bags with me. Just as you say, it happened when departing the train. Thank you, this is reassuring and our flight will have only been 3 hours from Paris, so we shouldn't be too tired or disoriented :) I may decide to choose the adventure then, and spend the money we saved on dinner the next night...

Good decision.

And with the money saved you can also go for a drink in the evening at "Couleur Local", a hidden restaurant bar (with a view of the Acropolis) in a small alley not far from Monastiraki and probably not far from your hotel. But don't go there for Greek food.

https://couleurlocaleathens.com/en/

Why not take the x95 BUS from airporot goes every 25 mins or so 24/7, costs only €6, virtually same time as Metro, goes to Syntagma Square. It's pickpocket-safe, comfortable seating, baggage piled in central "pen" inside bus only 2-3 stops (for airline staff) -- so u can relax, not clutch baggage or backpacks. On arrival instead of having to leap out of crowded trrain before door closes, you take your sweet time, because bus just sits there. Instead of toiling up from underground, step right off onto sidewalk, at top end of Othonos, of RH side of Syntagma (see map -- https://www.orangesmile.com/destinations/img/athens-map-big.jpg -- click to enlarge) Hotel 15 min walk from Monastiraki -- which direction?? If towards Acropolis, it may be near Syntagma (we always can give best help when people give specifics). For my hotel I roll my bag "downhill" on Amalias to Lysikratous ... 15 mins.... piece o' cake (and I'm way older than you guys). Of course, if it's in totaly other directions, You can get a taxi on Syntagma (be SURE of your address & ask est. fare before u get in)... shouldnt be more than €7 or so if in central athens. Or as others say, the flat-fee taxi (OR advance booking with City - Cab, same flat rate as those in taxi line.

Do NOT rent a car in Athens!! If you are traveling somewhere distant after Athens, maybe. but NOT for an Athens stay (and rent at airport & use bypass road -- no driving in center).

SO helpful! The hotel is the Altar Suites, which is west up Ermou from Monasteraki Square. Looks like the Monasteraki bus stop is closer to the hotel than the Monasteraki metro stop...

The X95 bus from the airport does not go to Monastiraki Square (the bus stops you mention are for other bus lines).
The Monastiraki metro station will be the closest to your Altar Suites hotel.

And as I assumed, you will be a 3 minute walk from "Couleur Locale"

I won't be able to help you for the next few hours, I'm leaving for... Athens :-)

Between customs and the possibility of having to check our bags on the flight from Paris

Coming from Paris there will be NO immigration/passport control to deal with, which would be the only processing delay. Customs should be a just walking out through the correct door. Waiting for checked bags likely to be the major consumer of time.

Getting door to door service with a taxi wouldn't be wasteful. Keep it simple.

All of this is so helpful! I have reached out to George's car service (thank you Rick) to see what they would charge door to door. Yes, simple is always good. And I would NEVER rent a car in Athens, yikes. Sounds like the metro or a car service are the best options since the airport bus doesn't get us close enough. I will include a cool stop to Coloure Locale...

The flat-rate charge for taxis between airport an anywhere in central Athens if you arrive before midnight is €40(about $42USD). In the evening, it takes about 45 minutes. (After Midnight until 6 AM the rate is €55) The 2 best-known car services charge the flat-rate charge -- Welcome Pickups and City Cab. Both can be booked online ... you are told the fee ahead, and pay by credit card. The driver meets you at luggage claim with a card with your name. All drivers are fluent in ENglish; a bonus is often a cold bottle of water. George's Taxi (which gained celebrity from constant promotion/praise by well-known website) will NOT state its standard airport-to-city fee, saying only "a few dollars more." I like services that state price up front.