I will say, who knows what boarding in January is going to look like! However, I will say we don't think the Key was necessary for priority boarding. We didn't wait at all and one sailing we got there around 10:30 and the other was around 12:00. On the Harmony, we thought the sit down lunch to avoid the buffet was really nice (on the Mariner, we had to sit with another family for lunch). We also dropped off our carry-on bags which was nice.
I was going to buy internet for my family of 4 anyway and the price of the Key was slightly more than that. Having the ability to text my kids was important to me and they would tell you having YouTube access was required because of the limited TV options. The idea of a lunch and breakfast made it worth it to us. We did get awesome seats for the water show. We didn't find that the activities were crowded, so we didn't need the special times.
For our trips now moved to 2021, I'm comparing the cost to the internet for one of them and if it's close, I'll probably get the Key again. For the other trip, I'm not because I don't think it's needed.