How to build a custom thread list for display in message-list-panel

I've been trying to build a custom component that will display only unanswered threads authored by paying customers.  Our paying customers have a role assigned to them "customer", so I'm querying the REST API for a list of unanswered posts, then identifying the author of each post, querying the roles of said author, then if they have the correct role, I want to append that thread to my sequence variable, which I will feed to the "forums.widget.message-list-panel" component for display.

Here's my code snippet:

<#list rest("/search/messages?openresponse=true&sort_by=-topicPostDate&page_size=" + results_list_size).messages.message as message>
    
  <#assign author = message.author.login>
    
  <#assign authorID = restadmin("/users/login/${author}/id").value>
    
  <#list restadmin("/users/id/${authorID}/roles").roles.role.name as authorRole> 
    <#if authorRole?? && (authorRole == "customer")>
      
      <#assign messagelist = messagelist + message>

When I save this code, Studio throws this exception:

FreeMarker template error

Only elements with no child elements can be processed as text.
This element with name "message" has a child element named: thread

The failing instruction (FTL stack trace):
----------
==> #assign messagelist = messagelist + m...  [in template "preview" at line 27, column 7]
----------

But if I change my message variable to drill down to the "thread" node level, by changing this line:

<#assign messagelist = messagelist + message>

to this:

<#assign messagelist = messagelist + [message.thread]>

it complains that my right-hand operand is not convertible to a string:

For "+" right-hand operand: Expected a string or something automatically convertible to string (number, date or boolean), but this evaluated to a sequence (wrapper: f.t.SimpleSequence):
==> [message.thread]  [in template "preview" at line 27, column 44]

The failing instruction (FTL stack trace):
----------
==> #assign messagelist = messagelist + [...  [in template "preview" at line 27, column 7]
----------

What am I doing wrong?  In other languages, this is just an array that I'm trying to append to, but Freemarker doesn't seem to like that.

Ultimately, I just want to display this custom-built list of unanswered threads with this code:

<div class="lia-panel lia-panel-standard top-five-threads-with-no-replies-wrapper">
	<@component id="forums.widget.message-list-panel" title="${message_list_title}" messages="rest_v1:"+messagelist style="wide" numMessages="conv:"+results_list_size />
      </div>