For reference: We ended up with this work-around with a "FormsResponsePage".

@inherits Umbraco.Web.Mvc.UmbracoViewPage<ContentModels.FormsResponsePage>
@using Umbraco.Forms.Core.Enums
@using ContentModels = Umbraco.Web.PublishedModels;
@functions
{
    private void WorkflowExecutionStatusCheck()
    {
        Enum.TryParse<WorkflowExecutionStatus>(
            (string)Session[nameof(WorkflowExecutionStatus)], 
            out var status);

        switch (status)
        {
            case WorkflowExecutionStatus.Completed:
                break;
            case WorkflowExecutionStatus.Failed:
                throw new Exception(
                    $"{nameof(WorkflowExecutionStatus)}: {Enum.GetName(typeof(WorkflowExecutionStatus), status)}");
            case WorkflowExecutionStatus.NotConfigured:
                break;
            case WorkflowExecutionStatus.Cancelled:
                break;
            default:
                throw new ArgumentOutOfRangeException();
        }
    }
}
@{
    WorkflowExecutionStatusCheck();
}

Though the customer already have a license, Umbraco Forms are simply not suited for anything more advanced when using it like once a year, and building custom controls just isn't viable in such a scenario.

NB! The code is a stub.