Hi JoAnne!
- You are correct, no guided tours/tour groups are allowed inside until June 9th. The reservation system for tour guides/tour groups will open on May 15th.
- There's a free mobile app that you can download from the Apple Store, Google Play store, etc.
- There's also an audio-guide that you can purchase for €6 at the visitor services desk (it's on the left side, as you enter Notre Dame). This audio-guide is a more spiritual/faith-based guide that was created in partnership with the Diocèse. There's nothing wrong with this! I'm simply stating that it's less focused on the topics relating to the architecture, the restoration techniques, etc.
- If you'd prefer a more historical/architectural audio-guide, I highly suggest "Notre Dame Whispers". You can download it for free on the Apple Store or Google Play store. This app/audio-guide was developed by the "Acoustics team", one of the nine scientific research teams working with the National Centre for Scientific Research, who are working on the restoration. This app/audio-guide is of the exterior, but I think it offers some unique perspectives that are not offered in other audio-guides of Notre Dame. I know this team personally, and I also genuinely really enjoy this audio-guide!
- Notre Dame opens at 7:50am on weekdays and at 8:15am on weekends. The first visitor time slot of the day that's offered is 9:00am, due to Morning Mass beforehand (8:00am on weekdays/8:30am on weekends). Visitors are still allowed to enter before 9:00am by waiting in the "Access without reservations" queue (blue signage), that's on the left side (if you're facing/looking at Notre Dame). Priority entrance is first given to those who are attending Morning Mass, then visitors are allowed to enter.
- For the lowest crowds, I recommend visiting on Tuesday, Wednesday or Thursday before 10:30am. If you're available later in the day on Thursday, Notre Dame is open late on Thursday nights, and this is generally a more peaceful time to visit! Notre Dame is open until 10:00pm on Thursday, but the ambulatory/the back chapels close at 9:30pm and everyone has to be out before 10:00pm, so I recommend visiting between 7:00pm and 9:00pm (the "sweet spot" is 8:00pm/8:30pm).
- The entrance to the Treasury is inside Notre Dame, on the south side. There is an entrance fee of €12 per person, since the Treasury is considered a museum. There are no online time slots/reservations, tickets are available on-site at the Treasury. The Treasury is open from Monday to Saturday from 9:30am until 6:00pm (until 9:00pm on Thursdays when Notre Dame is open late) and on Sunday from 1:00pm until 5:30pm. FYI, they start closing 30 minutes before actually noted closing time.
As for "Anything else you think I should know"...Well, that's a loaded question! I joke, but it's true, that "I cannot physically shut up about Notre Dame", haha. (I'm also the author of the Reddit post that Mardee linked in her comment above). But if you have any other specific questions, I'd be happy to help!