Can I set variable monthly payment amounts through PayPal REST API's billing plan/agreement? On existing agreements? - recurring-billing

Question

I entered this as an issue on the PayPal-PHP-SDK github but it's rather time sensitive at this point.
Our goal is a subscription service that monthly charges users a value they select for each content update during that month. A $1 subscriber would pay $8 if there were 8 updates, for example.
Our implementation so far (which is already live with a number of subscribers) is using the PayPal-PHP-SDK / REST API to create a Billing Plan for each subscriber with a payment definition set to an amount calculated to be the maximum potential monthly charge. I then expected to be able to use Agreement->setBalance() to lower the value to the actual intended charge and then Agreement->billBalance() to process the payment.
Unfortunately due to time pressures we launched without verifying this was a viable implementation, and I've discovered that those functions are only for unpaid/delinquent balances. The start_date on our plans is April 1, and we'll have had 4 content releases but we're set to charge our subscribers for the maximum possible 9 updates.
I've tried a variety of Agreement->update() and Plan->update() calls to change the monthly value, along the lines of:
$Patch = new PayPal\Api\Patch();
$Patch->setOp("replace")
->setPath("/payment_definitions/0/amount/value")
->setValue($patch_value);
$PatchRequest = new PayPal\Api\PatchRequest();
$PatchRequest->addPatch($Patch);
$Plan = PayPal\Api\Plan::get($plan_id, $apiContext);
$Plan->update($PatchRequest, $apiContext);
which returns an exception with "validation_error: Invalid Path provided". Attempts to json encode the data path, up to and including the entire Plan object with $Patch->setPath("/") instead give an exception with "MALFORMED_REQUEST - Incoming JSON request does not map to API request". I suspect most values cannot be updated on an executed agreement or activated plan.
However, I see through the merchant account's website that the monthly value can be manually updated, so I hold out hope that I'm simply going about my API requests the wrong way.
I've also tried creating a Payment object with Transaction->setPurchaseUnitReferenceId($AgreementId) since I'd read something about reference transactions being a potential solution, but I'm getting the same malformed request error:
$Item = new PayPal\Api\Item();
$Item->setCategory('DIGITAL')
->setPrice($pledge_value)
->setDescription($update_name);
$ItemList = new PayPal\Api\ItemList();
$ItemList->addItem($Item);
$Amount = new PayPal\Api\Amount();
$Amount->setCurrency('USD')
->setTotal($pledge_value);
$Transaction = new PayPal\Api\Transaction();
$Transaction->setPurchaseUnitReferenceId($AgreementId)
->setDescription($update_name)
->setAmount($Amount)
->setItemList($ItemList)
->setNotifyUrl($notify_url);
$Payer = new PayPal\Api\Payer();
$Payer->setPaymentMethod('paypal');
$RedirectUrls = new PayPal\Api\RedirectUrls();
$RedirectUrls->setReturnUrl($return_url)
->setCancelUrl($cancel_url);
$Payment = new PayPal\Api\Payment();
$Payment->setIntent('sale')
->setPayer($Payer)
->setRedirectUrls($RedirectUrls)
->addTransaction($Transaction);
$Payment->create($apiContext);
So is there a way to fix our current implementation and plans/agreements? Failing that, is there a REST API solution that I should have used / can try to migrate to? I'd like to avoid Classic API if possible.