You’re trying to mimic hand held. So 2-3 feet away and at or above your head (the approx length of your arm). So 3 feet seems about right for the longest, 2 feet for the shortest.

You do not want it looking up your nose. Horrible angle.

The gopro gooseneck is way too short at 8” for example.

A backpack mount is going to be on or right below your collar bone. Put on your backpack and you’ll see what I mean, so it needs to bend up a little bit

Try this https://www.amazon.com/Flexible-Gooseneck-Fitfort-Crosstour-Campark/dp/B07KB5L36J/ref=sr_1_7?crid=19NOZVHDU36S8&keywords=gopro+flexible+arm&qid=1662293442&sprefix=Gopro+flexible%2Caps%2C91&sr=8-7

What I’m worried is I bet there isn’t a backpack mount on the market that can hold that much weight horizontally.

You might just need to do handheld for self shots and if so, get this

https://www.amazon.com/Sametop-Backpack-Compatible-Session-Cameras/dp/B01BSE454S/ref=sr_1_4?crid=19DW2XB7C4TRI&keywords=Gopro+clip&qid=1662293667&sprefix=gopro+clip%2Caps%2C83&sr=8-4