Well, I went to DLP New Year's Eve party on 2019-2020, and it was OUT OF THIS EARTH. I've had new year at WDW and many places, but I have to say I just loved DLP's. However, it is important to say that the BEST of it all was the party that started after the fireworks, at around 00:30 and lasted until 2:00 am. Disney fan DJ "Afrojack" offered an incredible concert with special effects and projections over the castle, using fire, lasers, lights and the fountains. It was just incredible. In fact, we'll be back this year, but I doubt they'll have such an incredible DJ with such a display.
The party is still available but usually sells out by november, leaving only some dinner packages availables. If you are only for the party, you can enter after 18:00. If you have both the ticket and the party ticket, you can just remain in the park. The dining packages are incredibly expensive (350+ euros), but they include the ticket. That day all restaurants have a "higher priced menu" for the ocassion, and those non-table service restaurants, well... they'll be packed.
On 2019, DLPs last NYE, they also had a New Year party included with the park entrance over at the Walt Disney Studios park, with fireworks and projections over the Hollywood Tower Hotel. I didn't see it, as I was over at the DLP NYE Party but it was announced everywhere. At around 2:00 pm EVERYTHING closes, Disney Village, parks, restaurants, etc.
They mount up Champagne bars around both parks, specially in the
Disneyland park, and they offer the glasses at cheaper prices than in the restaurants (10 euro / glass), they also offer "sparkling apple juice" as a non-acoholic option, so you'll be able to meet the new year with a champagne glass in your had, no matter where you are.
In Disney Village they have a small stage and have music and also have a "small" NYE party.
If you're staying on a DLP hotel, you get discount on the party, so that's a no-brainer, plus you already have the standard tickets included.
If you're staying offsite, then I would reccomend my plan on 2019: get a single park ticket and the party. Have a good breakfast, spend your day at Walt Disney Studios park, make a reservation for a late-lunch at Chez Remy's (the restaurant at the exit of the Ratatouille ride) and have a nice and relaxed lunch, at 18:00 head over to DLP for the party and enjoy rides, parades, etc, don't forget to have a snack before midnight if you're planning on drinking champagne so it doesn't hit you.
As for the health rules, like Karin said, if you have double vaccination, you're good to go.
I would recommend the party without doubt, but I'm not sure how good this year will be during the aftermath of the pandemic. But judging from my 2019 experience, it was the best Disney (or non-Disney) NYE party ever, if you like music and party, at least. Heheh.