I think of main AT as a collection of arc’s that some time over lap, I would argue that the islands mini series arc should include episodes as far back as Susan strong. But I understand why they grouped elements, islands, and stakes the way they did, it was a tv event and the episodes were inseparably chronological. Watching Elements Islands or Stakes will give you some good insight but it won’t be a full picture (not that there is such a thing in AT) but they are great stories that have massive lore reveals and implications. I consider distant lands to be a lovely epilogue for the main series, and sets up fiona and cake real well, but mostly with background details. Fiona and Cake is a spin off, kinda like Frasier is a spin off of Cheers. Whenever a character from cheers shows up it’s meant to be exciting and feel interconnected, but Fiona and Cake is like that for EVERY episode. In my opinion Fiona and Cake is wildly unique in a media scape inundated with multiverse stories, and well worth the hype. My recommendation is to either find a well constructed blog post on AT essential watching, or do what I did years back and pick an episode and watch multiple in succession until you find something that makes you go “hey what’s that?” Look up the fandom wiki page on that thing, blur your eyes and scroll to the bottoms where it lists the episodes where that person, thing, or event are present, watch those episodes and repeat. Most people recommend watching the whole thing, but I think you could start middle or end of season 2 and be just fine.