POC to insert a feeditem as a guest user on salesforce

AccountHandler.cls.java

public class without sharing AccountHandler {
  public static void postToChatter() {
    FeedItem fi = new FeedItem(/*Add required fields*/);
    insert fi;
  }
  
  @future
  public static void asyncPostToChatter() {
    postToChatter();
  }
  
  public class PostToChatterQueueable implements Queueable {
    private List<Account> newAccounts;
    
    public PostToChatterQueueable(List<Account> newAccounts) {
      this.newAccounts = newAccounts;
    }
    
    public void execute (QueueableContext ctx) {
      AccountHandler.postToChatter();
    }
  }
}

AccountTrigger.trigger.java

trigger AccountTrigger on Account (after insert) {
  if (Trigger.IsAfter && Trigger.IsInsert) {
    // Synchronous
    AccountHandler.postToChatter();
    // Async Future Method
    AccountHandler.asyncPostToChatter();
    // Async Queueable
    System.enqueueJob(new AccountHandler.PostToChatterQueueable());
  }
}