FeedItemViewModel.cs

public record class FeedItemViewModel(string Author, string Text, long Likes, long Reposts, long Replies);

var items = timelineData.Feed
  .Select(f => f.Post)
  .Where(p => p is not null)
  .Select(post => new FeedItemViewModel(
    post!.Author?.DisplayName ?? "",
    (post.Record as Post)?.Text ?? "",
    post.LikeCount ?? 0,
    post.RepostCount ?? 0,
    post.ReplyCount ?? 0)).ToList();