I am a bit stuck on trying to solve this

So I have a webapp, and decided to add a blog section using wagtail. This means I have quite a few pages and models already preexisting, that I do not want to touch, as well as some users already. I am aware of the owner field in the base Page class, but can't see a way to extend to into a m2m field.

To achieve this, I have created a custom user model at users.User. For blog community purposes, I wanted to add the option of picing an avatar and setting a biography, so I added those two (avatar and bio are newly added fields)

class User(AbstractUser):
    name = models.CharField(_("Name of User as displayed to other users"), blank=True, max_length=255)
    company = models.ForeignKey(Company, null=True, on_delete=models.CASCADE)
    job_title = models.CharField(max_length=100)
    avatar = models.ForeignKey('wagtailimages.Image', null=True, blank=True, on_delete=models.SET_NULL, related_name='+')
    bio = models.TextField(max_length=300, blank=True)

Then I have a classic Blogpost page, with title, body streamfield etc... However, I am trying to replace something where you can add multiple post editors, and each of them has edit rights. So I tried something like this:

First an orderable since I need many2many:

class PostAuthorsOrderable(Orderable):
    page = ParentalKey("blog.BlogPost", related_name="post_authors")
    author = models.ForeignKey("users.User",on_delete=models.CASCADE)

    panels = [
       SnippetChooserPanel("author"),
    ]

And then in the BlogPost page itself:

class BlogPost(Page):
    ...
    content_panels = Page.content_panels + [
        MultiFieldPanel(
            [
                InlinePanel("post_authors", label="Author", min_num=1, max_num=4)
            ],
            heading="Authors (don't forget to add yourself!)"
        ),
        ...
    ]

However this is not working, any idea on how to achieve this? I do get the proper form in the wagtail admin, but I get an error. It tries to do a callback, presumable to find a list of all possible users to select, but it is giving a 404, presumably because the user is not registered as a snippet.

Before I try registering the user as a snippet, I wanted to get a second opinion here, because that feels a bit wrong somehow

PS: bonuspoints if you can also tell me how I can get a widget in the editor to just select authors by autocompleting their names into a text field , a bit like this: example

EDIT: When I click the button to add an Author, nothing happens, all I can see is a 404 in the console

172.19.0.1 - - [17/Nov/2021 15:26:09] "GET /admin/snippets/choose/users/user/ HTTP/1.1" 404 -
Not Found: /admin/snippets/choose/users/user/