So I think I have honed in on the issue with template previews not showing up in Nextcloud. The app is looking for a template in the /app/richdocuments/template/preview folder. The js that puts the document there at creation of a new template is
* Append a new template to the dom
*
* @param {Object} data the template data from the template controller response
*/
function appendTemplateFromData(data) {
var template = document.querySelector('.template-model').cloneNode(true);
template.className = '';
template.querySelector('img').src = data.preview;
template.querySelector('figcaption').textContent = data.name;
template.querySelector('.delete-template').href = data.delete;
document.querySelector('#richdocuments-templates > ul').appendChild(template);
template.querySelector('.delete-template').addEventListener('click', deleteTemplate);
};```