ah i found it, leaving here for anyone doing similar stuff. My controller was wrong, i was awaiting the content, but you can just directly return the promise (bit commented out was before):
async completion(ctx) {
return await strapi.plugin('openai-assistant').service('assistantService').completion( ctx);
// const content = await strapi.plugin('openai-assistant').service('assistantService').completion( ctx);
// ctx.body = { content };
}