The first cutscene of the campaign is kinda a very quick summary of the relevant events of the past few expansions. There are videos made by popular content creators online that cover the story in more detail (you don’t need to watch the 10 hour one, other content creators have made shorter ones).
There is an in game timeline, which tells you the general plot in 2 or 3 sentences and allows you to play a short mission from the DLC. You can find in the director on the top right.
Seasons are excluded though so the best way to get up-to-speed might My Name is Byf recent 'story up to final shape' video. Its 10 hours long, but the description has timestamps for the revent seasons so you can skip around.
Yes and no. I love me some long form videos. 10 hours might be much. I played through the catch up missions on the timeline last night and got through a mission or two from Final Shape. I'm worried I fall back into the same issue as before, so much content is relevant and not at the same time. Like I need X weapon or exotic from a dungeon, but I need to run Y raid to get gear to do it. But I'm not leveled for raid, so I need to run strikes. But now strikes are recommended at 1945 light, which I don't think I am yet, so I gotta do random shit til I can run strikes, to run a raid, to run a dungeon, to get into the content.
Not that that is the case, it's just so much to consider with minimal knowledge going in.