When does my account downgrade after cancellation

You have to cancel in a smart way with one eye on the renewal date to avoid an early cancellation fee.

As mentioned, Adobe does not permit scheduled cancellation in advance. It's all down to your timing.

does this happen at the renewal interval next year or does this happen immediately thus loosing a years worth of paid for subscriptions?

Service continues until the end of the monthly billing period in which you cancel.

See: https://www.adobe.com/legal/subscription-terms.html

However, keep in mind the terms around early cancellation.

If you cancel anywhere in months 1 through 11 of your annual contract then "...you’ll be charged a lump sum amount of 50% of your remaining contract obligation and your service will continue until the end of that month’s billing period."

The early cancellation fee is a sliding scale depending on when you cancel.

See https://helpx.adobe.com/manage-account/using/creative-cloud-subscription-terms.html

If you cancel in month 12 then there is no cancellation fee.

Note the renewal date in your online account.

https://account.adobe.com/plans

Set a reminder on your smartphone or Google Calendar to cancel in the 12th month, after payment is processed but before it automatically rewnews for another year.

Hope that helps.

because you can't schedule cancellation with adobe, set a reminder to cancel within a few days of your renewal date.

It happens when you cancel.