Passion Monuments Subscription

or it might be like here in the UK when places are shut for the winter and don't open until Easter?

It would depend on the place you're looking at. If a museum they may have scheduled closure for whatever reason, like a new installation, or minor renovations of a space, etc. If a seasonal place, like Giverny, it make be closed those months. Or, it just may be that they don't open reservations until a short time before. Hope you hear back from the places you've emailed.

As for the subscription pass, I've not heard of it so I have no opinion on that.

Here's the website for the Centre des Monuments Nationaux:

https://www.monuments-nationaux.fr/en/passion-monuments-subscription

The cost is 45 euros for an individual or 70 euros for an individual who can take a guest. I've scanned the list of 93 covered sights, and I do recognize some of them, but they are scattered all over the country. I suspect you'd need quite a long trip, or a rental car and a strong interest in chateaux and/or church towers, to save a lot of money, but modest savings might be possible for other visitors, perhaps even folks going only to Paris, if their interests align well with the covered sights.

Oh good, it offers a visit to the Notre Dame Towers. That alone makes me dubious about its usefulness.

I think it says they're closed (or something to that effect; I don't really know French).

Thanks all, I did here back from one of the sites we're interested in, the cathedral in Reims - they said they're booking calendar opens gradually but they will be open the day we want to visit so I should check back a week before the visit to book the entry.
As for the pass and some of the sites being closed, i.e. Notre Dame, that's true so I'll just keep investigating the places we do want to see.
I'll also have to keep track of how far in advance we need to reserve entry times, maybe for this time of year it's not as much of a concern.

Not many places actually require advance booking (basically just some of the places you'd already heard of) unless you want to take a tour; those can be limited-capacity. The only way to be sure is to check each website, though.

We recently picked up the Passion Monuments subscription and it seemed like a good deal to us and we will make good use of it as we are visiting other parts of France, including Mont Saint-Michel. There were many places we wanted to visit anyway, so we are buying the DUO at 35 euros pp, therefore, we should save quite a bit over the Paris Museum Pass and we have a full year to use it (rather than the 4 day limit of the 77 euro museum pass).

Each site on their own adds up as follows:
Mont Saint-Michel - 16 Euro
Arc de Triomphe - 16 Euro
Saint Chapelle - 14 Euro (20 for a pair)
Pantheon - 14 Euro
Conciergerie - 13 Euro
Total = 70 Euro each (so we are pretty much at half price)

I was hoping Notre Dame would open by time we are there in late spring, but seems they have now delayed opening it to end of 2024. Too bad, was hoping the Olympics would have helped. Maybe I will make a trip back to Paris before the year's pass runs out. :)