So based on the responses and what research I could do (it is overwhelming for a rookie), I have come up with a plan. I would love all thoughts on the plan and possible adjustments.
April 18 (Fri): Fly in to CDG, make way to housing, wander & eat
April 19 (Sat): Louvre in am (reserve ahead), Notre Dame, Arc de Triomphe
April 20 (Sun): Versailles in am (reserve ahead), cruise on Seine in pm
April 21 (Mon): Wander, shop, relax OR Pantheon & Luxembourg Gardens
April 22 (Tue): Catacombs in am, wander OR Pantheon & Luxembourg Gardens
April 23 (Wed): drive rented car to Amboise for the rest of our stay
Hi wvovolvo!
If you want to visit Notre Dame as a visitor/tourist (and not attend Mass/liturgical ceremonies), I would strongly recommend moving your visit to Notre Dame to be on Tuesday, April 22nd instead.
Notre Dame from Thursday, April 17th until Tuesday, April 22nd is going to be exceptionally busy due to Easter Celebrations. Easter is one of the busiest times of the year at Notre Dame, and it will be the first Easter after the reopening. Monday, April 21st (Easter Monday) is also a Public Holiday in France. Plus, all the schools in France will be on Spring Break, and tourism season will be picking up.
Another reason why I recommend visiting Notre Dame on Tuesday, April 22nd is because it's quite possible that between Thursday, April 17th until Tuesday, April 22nd, no advanced reservations/time slots will be offered due to the Easter Celebrations (this hasn't been 100% confirmed yet, but it was like this on Christmas Eve & Christmas Day, there were no advanced reservations/time slots offered). Notre Dame has a very strict capacity limit and those attending Mass/liturgical ceremonies (ie. Easter Celebrations) always get priority entrance over visitors/tourists.
Based on the days you'll be in Paris, the best day/time to visit Notre Dame (with the lowest crowds) will be on Tuesday, April 22nd with a time slot between 9:00am and 10:00am, so that you'll be able to see everything (explanation following), and still beat the heavy crowds! Notre Dame opens at 7:50am on weekdays and at 8:15am on weekends, however, the front section of the Nave (the centre) and the crossing of the Transept are closed to visitors until approximately 9:00am (once Morning Mass is finished).
Time slots can be reserved on Notre Dame’s free online reservation system for dates up to 2 days in advance: https://resa.notredamedeparis.fr/fr/reservationindividuelle/tickets
The first batch of new time slots is released at midnight (Paris time), for the date 2 days ahead. For example: At midnight (Paris time) on April 1st, time slots are released for April 3rd. Any dates beyond April 3rd will automatically be greyed out/appear to be full. This first batch can fill up within ~20 minutes of being released, so I recommend opening the reservation system around 11:50pm. For the fastest connection, I recommend accessing it on your computer using Chrome.
New/additional "same day" time slots are released 4 hours in advance. For example: At 5:00am (Paris time), new time slots are released for 9:00am for that same day. At 5:30am, new time slots are released for 9:30am, etc. However, there are certain hours when no time slots are ever offered, therefore no new time slots are released 4 hours in advance of those hours. For example: No time slots are offered for 12:00pm, therefore no new time slots will be released at 8:00am. The availability of these time slots is based on the planned and/or current capacity available inside the cathedral, and are not guaranteed.
New/additional "spontaneous" time slots are released sporadically throughout the day. The availability of these time slots is based on the current capacity available inside the cathedral, and are not guaranteed.
If you have any questions, just let me know!