I would like a way to do this with the partial tag helper.

LayoutForPartialTagHelperExample.md

What I'm looking for is a nicer more declarative approach to Templated Razor Delegates

In Any old Razor Page or View - SomePage.cshtml

...
<div>
  <partial name="Shared/_PartialLayout" for="SomeModel">
    <strong>Content</strong> that will be injected into the layout, which 
    will get rendered here.
  </partial>
</div>
...

In _PartialLayout.cshtml

<div class="stuff">
  <h2>@Model.SubTitle</h2>
  <div class="passed-in-content">
    @RenderContent()
  </div>
</div>

And the rendered result

...
<div>
  <div class="stuff">
    <h2>@Model.SubTitle</h2>
    <div class="passed-in-content">
      <strong>Content</strong> that will be injected into the layout, which 
      will get rendered here.
    </div>
  </div>
</div>
...

The actual problem domain is I want to create reusable dialogs. I'm making use of this web component: https://github.com/github/details-dialog-element. But there's a lot of boilerplate to use it in a real app. My quick and dirty solution is to put much of that boilerplate in a layout, then create a partial per dialog invocation. The latter part is a pain.

DialogLayout.cshtml

@{
    var dialogModel = ViewData[nameof(DialogModel)] as DialogModel
        ?? throw new InvalidOperationException("Add a DialogModel to ViewData");
    var summaryCssClass = dialogModel.ButtonLabel is null
        ? null
        : "btn btn-primary";
    // If there's a button label, then the summary is a button to 
    // open the dialog. Otherwise, it's a dialog launched by some
    // other action, thus we set the tab index to -1.
    var summaryTabIndex = dialogModel.ButtonLabel is null
        ? "-1"
        : null;
    // If the dialog contains a from, we want to make sure it's tabbable,
    // otherwise we want to remove the tab index.
    var detailsTabIndex = dialogModel.ContainsForm
        ? null
        : "-1";
}

<details id="@dialogModel.Id"
    class="details-reset details-overlay details-overlay-dark">
    <summary
        tabindex="@summaryTabIndex"
        aria-label="@dialogModel.SummaryLabel"
        class="@summaryCssClass"
        role="button">@dialogModel.ButtonLabel</summary>
    <details-dialog class="anim-fade-in fast Box Box--overlay d-flex flex-column"
                    role="dialog"
                    aria-modal="true"
                    tabindex="@detailsTabIndex">
        <div class="Box-header">
            <button class="Box-btn-octicon btn-octicon float-right"
                    type="button"
                    aria-label="Close dialog"
                    data-close-dialog>
                <span class="iconify" data-icon="iwwa:delete" data-inline="false"></span>
            </button>
            <h3 class="Box-title">@dialogModel.Title</h3>
        </div>
        @if (!(dialogModel.WarningMessage is null)) {
            <div class="flash flash-full flash-error">
                <span class="iconify" data-icon="whh:warningsign" data-inline="false"></span>
                @dialogModel.WarningMessage
            </div>
        }
        <div class="Box-body overflow-auto">
            @RenderBody()
        </div>
    </details-dialog>
</details>

ChangeUsernameDialog.cshtml

@model Aboard.Boards.Pages.Settings.AccountPageModel
@{
  Layout = "Shared/Dialogs/_Layout";

  ViewData[nameof(DialogModel)] = new DialogModel {
    Id = "rename-form-dialog",
    Title = "Enter a new username",
    SummaryLabel = "Change username",
    ContainsForm = true
  };
}

<form method="post" aria-label="Change username" accept-charset="UTF-8">
  <div class="flash flash-error" asp-validation-summary="ModelOnly"></div>
  <div class="form-group">
    <input asp-for="Input.Username"
           class="form-control"
           autocomplete="off"
           spellcheck="false"
           autocapitalize="off"
           autofocus
           required
           aria-label="Username" />
    <label asp-for="Input.Username" class="note"></label>
    <span asp-validation-for="Input.Username" class="text-danger"></span>
  </div>
  <button type="submit" class="btn btn-block btn-primary" data-disable-invalid>Change my username</button>
</form>

Then when I need to invoke this.

<partial name="Shared/Dialogs/ChangeUsernameDialog" />

Lots of little challenges here. For example, this partial contains a form that posts to its containing page. So any solution needs to make that easy.