Template Controller

posts_list_controller.html

<template name="postsListController">
  {{> posts_list context}}
</template>

posts_list_controller.js

// see https://www.discovermeteor.com/blog/template-level-subscriptions/

/*

This template acts as the controller that sets and manages the reactive context 
for the embedded postsList template. It receives its parameters from a "caller" template.

*/


Template.postsListController.onCreated(function () {

  // 1. Initialization (*not* reactive!)
  var instance = this;

  // initialize the reactive variables
  instance.terms = new ReactiveVar(instance.data.terms);
  instance.postsLimit = new ReactiveVar(Settings.get('postsPerPage', 10));

  // 2. Autorun

  // will re-run when terms are changed, either by the router or by the template itself
  instance.autorun(function () {

    // get terms from data context
    var terms = Template.currentData().terms; // ⚡ reactive ⚡

    // get limit from local template variable
    var postsLimit = instance.postsLimit.get(); // ⚡ reactive ⚡

    // create new subscriptionTerms object using the new limit
    var subscriptionTerms = _.extend(_.clone(terms), {limit: postsLimit}); // extend terms with limit

    // use this new object to subscribe
    var postsSubscription = instance.subscribe('postsList', subscriptionTerms);
    var usersSubscription = instance.subscribe('postsListUsers', subscriptionTerms);

    var subscriptionsReady = instance.subscriptionsReady(); // ⚡ reactive ⚡

    console.log('// ------ autorun running ------ //');
    Tracker.onInvalidate(console.trace.bind(console));
    console.log("terms: ", terms);
    console.log("limit: ", postsLimit);
    console.log("ready: ", subscriptionsReady);

    // if subscriptions are ready, set terms to subscriptionsTerms
    if (subscriptionsReady) {
      instance.terms.set(subscriptionTerms);
    }
  
  });

});

Template.postsListController.helpers({
  context: function () {

    var instance = Template.instance();

    var terms = instance.terms.get(); // ⚡ reactive ⚡
    var postsReady = instance.subscriptionsReady(); // ⚡ reactive ⚡

    var postsLimit = terms.limit;
    var parameters = Posts.getSubParams(terms);
    var postsCursor = Posts.find(parameters.find, parameters.options);

    var context = {

      // posts cursor
      postsCursor: postsCursor,

      // posts subscription readiness, used to show spinner
      postsReady: postsReady,

      // whether to show the load more button or not
      hasMorePosts: postsCursor.count() >= postsLimit,

      // what to do when user clicks "load more"
      loadMoreHandler: function (instance) {
        event.preventDefault();

        // increase limit by 5 and update it
        var limit = instance.postsLimit.get();
        limit += Settings.get('postsPerPage', 10);
        instance.postsLimit.set(limit);

      },

      // the current instance
      controllerInstance: instance

    };

    return context;
  }
});

I switched back to the autorun pattern after all. Using Template.currentData() still lets us depend on the parent context though.