Microscopically, cystic spaces/lesions involves the deep portion of the appendiceal wall, some of which are close to the serosal surface.

Images 1-3: from the mucosal/luminal surface.

Images 4-6: from two cystic lesions close to the serosa.

Impression:  a large mucinous mass of the appendix. Microscopically, focal stratification is present, without significant nuclear atypia. Though the lesion involves the subserosal areas (the diverticuli-like changes are frequently in this tumor) , no destructive invasion is identified.

Diagnosis:

Mucinous adenoma with low-grade dysplasia or low-grade mucinous neoplasm: 阑尾低级别黏液性腺瘤

You are doing wonderfully.  If this case was perforated, it would be called as "borderline neoplasm with uncertain malignant potential", since it will cause uncureable recurrence later.