Done quite a lot more digging and hidden away on the documentation page is this beauty:

https://our.umbraco.com/documentation/Add-ons/UmbracoForms/Developer/Extending/Adding-a-Type#overriding-default-providers-in-umbraco-forms

So for anyone else who happens to have the same issue, you can override/extend existing workflow types. Here's some sample code to show you how I did it (beware of the session "hack" - it's the only way I could find out how to pass data to the view, not ideal but it does work).

Workflow Definition:

using System.Web;
using Umbraco.Core;
using Umbraco.Forms.Core;
using Umbraco.Forms.Core.Attributes;
using Umbraco.Forms.Core.Data.Storage;
using Umbraco.Forms.Core.Enums;
using Umbraco.Forms.Core.Persistence.Dtos;
using Umbraco.Forms.Core.Services;
using Umbraco.Web;

namespace Example.Core.WorkflowTypes
{
    public class SendRazorEmailWorkflowType : Umbraco.Forms.Core.Providers.WorkflowTypes.SendRazorEmail
    {
        #region Constructors

        public SendRazorEmailWorkflowType(IHttpContextAccessor httpContextAccessor, IFieldTypeStorage fieldTypeStorage, IFormStorage formStorage, IUmbracoContextAccessor umbracoContextAccessor, IPageService pageService, IWorkflowEmailService workflowEmailService)
            : base(httpContextAccessor, fieldTypeStorage, formStorage, umbracoContextAccessor, pageService, workflowEmailService)
        {

        }

        #endregion

        #region Settings

        [Setting("Email Introduction", Description = "Displayed before the records in the email body.", View = "TextArea")]
        public string EmailIntroduction { get; set; }

        [Setting("Email Body", Description = "Displayed after the introduction.", View = "TextArea")]
        public string EmailBody { get; set; }

        [Setting("Email Records Text", Description = "Displayed before the records are shown.", View = "TextArea")]
        public string EmailRecordsText { get; set; }

        [Setting("Email Outro", Description = "Displayed after the email body.", View = "TextArea")]
        public string EmailOutro { get; set; }

        [Setting("Email Link URL", Description = "Optional link, if set, a button will be displayed in the email.", View = "TextString")]
        public string EmailLinkUrl { get; set; }

        [Setting("Email Link Title", Description = "Optional link title, if the Email Link URL is set, this text will be used on the button.", View = "TextString")]
        public string EmailLinkTitle { get; set; } = "Learn more";

        [Setting("Show Records in Email", Description = "If enabled, the email will display the fields submitted.", View = "Checkbox")]
        public string ShowRecordsInEmail { get; set; }

        #endregion

        public override WorkflowExecutionStatus Execute(Record record, RecordEventArgs e)
        {
            HttpContext.Current.Session["emailIntroduction"] = EmailIntroduction;
            HttpContext.Current.Session["emailBody"] = EmailBody;
            HttpContext.Current.Session["emailRecordsText"] = EmailRecordsText;
            HttpContext.Current.Session["emailOutro"] = EmailOutro;
            HttpContext.Current.Session["emailLinkUrl"] = EmailLinkUrl;
            HttpContext.Current.Session["emailLinkTitle"] = EmailLinkTitle;
            HttpContext.Current.Session["showRecordsInEmail"] = ShowRecordsInEmail;

            if (EmailLinkTitle.IsNullOrWhiteSpace())
            {
                HttpContext.Current.Session["emailLinkTitle"] = "Read more";
            }

            return base.Execute(record, e);
        }
    }
}

View/Razor File (I've only included how to retrieve the data here, but you can use these variables however you want from there on out):

@inherits UmbracoViewPage<Umbraco.Forms.Core.Models.FormsHtmlModel>

@{
    //This is an example email template where you can use Razor Views to send HTML emails

    //You can use Umbraco.TypedContent & Umbraco.TypedMedia etc to use Images & content from your site
    //directly in your email templates too

    //Strongly Typed
    //@Model.GetValue("aliasFormField")
    //@foreach (var color in Model.GetValues("checkboxField")){}

    //Dynamics
    //@Model.DynamicFields.aliasFormField
    //@foreach(var color in Model.DynamicFields.checkboxField

    //Images need to be absolute - so fetching domain to prefix with images
    var siteDomain = HttpContext.Current.Request.Url.GetLeftPart(UriPartial.Authority);
    var assetUrl = siteDomain + "/assets/images";

    var emailIntroduction = HttpContext.Current.Session["emailIntroduction"];
    var emailBody = HttpContext.Current.Session["emailBody"];
    var emailRecordsText = HttpContext.Current.Session["emailRecordsText"];
    var emailOutro = HttpContext.Current.Session["emailOutro"];
    var emailLinkUrl = HttpContext.Current.Session["emailLinkUrl"];
    var emailLinkTitle = HttpContext.Current.Session["emailLinkTitle"];
    var showRecordsInEmail = HttpContext.Current.Session["showRecordsInEmail"];
}

Thanks,

Ben