Trusty old ChildRelationship to the rescue:

for (ChildRelationship relation : SObjectType.Topic.getChildRelationships())
    system.debug(relation);

Relevant relationships and properties:

[getChildSObject=FeedComment;getField=ParentId;getRelationshipName=null;]
[getChildSObject=FeedItem;getField=ParentId;getRelationshipName=null;]

You should just be able to do:

SELECT Body, BestCommentId, CommentCount, LikeCount, Type,
    (SELECT CommentBody FROM FeedComments)
FROM FeedItem WHERE ParentId = '<topic_id_here>'

If you want to filter on multiple Topic parents, you could change your WHERE clause to:

ParentId IN ('<topic1_id>', '<topic2_id>', 'etc.')

Trusty old ChildRelationship to the rescue:

for (ChildRelationship relation : SObjectType.Topic.getChildRelationships())
    system.debug(relation);

Relevant relationships and properties:

[getChildSObject=FeedComment;getField=ParentId;getRelationshipName=null;]
[getChildSObject=FeedItem;getField=ParentId;getRelationshipName=null;]

You should just be able to do:

SELECT Body, BestCommentId, CommentCount, LikeCount, Type,
    (SELECT CommentBody FROM FeedComments)
FROM FeedItem WHERE ParentId = '<topic_id_here>'

If you want to filter on multiple Topic parents, you could change your WHERE clause to:

ParentId IN ('<topic1_id>', '<topic2_id>', 'etc.')

You can also do a Left Inner Join to see which records have a relevant TopicAssignment:

SELECT ... FROM FeedItem WHERE Id IN (
    SELECT EntityId FROM TopicAssignment
    WHERE EntityType = 'FeedItem'
    AND TopicId IN ('<id1>', 'etc.')
)