I really don't know whether it's possible to downgrade from advanced or not. I agree the .ca advanced form has become quite a jumble, they've added-on a ton of extra stuff these last few years. My response has been to use the "Customize" link located at the top right corner of the page to hide a lot of elements I don't need or use.
If you use a tablet or mobile phone another option would be to download the app, that listing tool is very basic. Personally I hate working with touch screens but some don't seem to mind. Ebay has developed a workable replacement for the classic advanced listing tool, it has been available on DOTcom for ages ... maybe they'll bring it to DOTca in 2022.
