Powershell - Create a JSON object with arrays - System.Collections.Hashtable issue

Question

I want to create a JSON object in powershell, for sending messages to slack from a build pipeline. The JSON I will be creating looks like this :

    {
   "blocks":[
      {
         "type":"header",
         "text":{
            "type":"plain_text",
            "text":"Release completed"
         }
      },
      {
         "type":"divider"
      },
      {
         "type":"section",
         "fields":[
            {
               "type":"mrkdwn",
               "text":"*Release number*\nRelease-17"
            },
            {
               "type":"mrkdwn",
               "text":"*User*\nJohn.Smith"
            },
            {
               "type":"mrkdwn",
               "text":"*Source version*\n20230110.1"
            }
         ]
      }
   ]
}

So it has an array in there, and a few things that I am struggling with! So far, I am doing the following :

$jsonBase = @{}
$blocksList = New-Object System.Collections.ArrayList #this is the main 'blocks' array

$fieldsList = New-Object System.Collections.ArrayList #create a list for the 'fields' section
$fieldsList.Add(@{"type"="mrkdwn";"text"="*Release number*\nRelease-17";})
$fieldsList.Add(@{"type"="mrkdwn";"text"="*User*\nJohn.Smith";})
$fieldsList.Add(@{"type"="mrkdwn";"text"="*Source version*\n20230110.1";})

$textData = @{"type"="plain_text";"text"="Release completed";} #create an object for the header text
$dividerData = @{"type"="divider";} #create an object for the divider

$blocksList.Add(@{"type"="header"; "text"=$textData})
$blocksList.Add(@{"type"="divider"})
$blocksList.Add(@{"type"="section"; "fields"=$fieldsList;})

$jsonBase.Add("blocks",$blocksList)

write-output $jsonBase | ConvertTo-Json

But my outputted JSON is not correct at all!

{
    "blocks":  [
                   {
                       "text":  "System.Collections.Hashtable",
                       "type":  "header"
                   },
                   {
                       "type":  "divider"
                   },
                   {
                       "fields":  "System.Collections.Hashtable System.Collections.Hashtable System.Collections.Hashtable",
                       "type":  "section"
                   }
               ]
}