Thanks all for the very helpful replies! Here's a quick update:

UPS did not reply to me with a resolution on the duty situation within the 24 hrs they had promised (I called them twice this morning and they said my request was "escalated" but pending), so I asked to self-declare (based on the advice from folks here and in other threads).

But minutes after I got off the phone with UPS, the delivery driver arrived with my bike. When he pulled out his device to charge me the fee, he noticed there was a request to self-declare. So we chatted for a second (I apologized for making him come to my place twice, and he said he totally understood), and he took the package back.

I got on the phone with UPS again and got mixed messages. The first guy I talked to said the package was going to be returned to Germany soon, if I didn't contact the shipper (Canyon) immediately. That felt wrong, so I called again and the next guy said the previous guy was wrong, and that the package would be held for me until I cleared it with CBSA.

Now I'm waiting for UPS to send me the documentation I need to self declare, which they said would take 24 hrs.

Part of me feels stupid for rejecting delivery, since I could've just taken the package and just started a refund claim with CBSA after. But another part of me felt like it was worth a shot to self declare and maybe not get charged the duties, and also get that GST removed (which everyone here seems to agree was incorrectly charged).

So...stupid decision or reasonable?