Back in the day you could get ranked fairly easy by just spamming. Now it takes more of a step by step approach in order to get ranked.
I actually have a few backlinks from Bing aimed at one of my websites
You just have to think outside the box sometimes in order to get those golden links which will help your site out.
Your 5th method "Reverse Engineering (Competitor Link Analysis)" is a great way to stay on top of your competitors. I'm pretty sure our competitors get a little mad when they see we get the same links as they do lol.
An easy way to figure out your competitors backlinks is to use ahrefs