I would personally have no issue building a credit line with Amazon by sending large chunks at a time rather than on every single purchase.

Yeah, definitely could see that being viable. A $20 fee to get a 1k line of credit isn't as unreasonable, and I think folks would go for that.

There's also the ability to borrow against the Bitcoin which is probably the best option.

Yeah, for now at least that does feel like it'd be the best option. That hinges on nobody in power deciding it's time to tax unrealized gains though, as, once that happens I think any lending against an asset like BTC (or traditional stocks/etc) becomes unwieldy pretty quickly. Which I only have in my mind because I recently watched some fool argue for taxing unrealized gains and I was tempted to nope out of the internet for the rest of the day lol. Currently borrowing against it as you said would be the best option for everyone in a world where we were using BTC as a fiat-like base.