...i'm trying to put off buying a new pc as long as possible.
There's nothing to stop you putting BiaB on an additional disc, whether USB or internal. For USB, you'll likely need a USB 3.x port on the PC (the ones with the blue tab inside).
Adding a disc inside a desktop machine is usually pretty easy.
Laptops can be less easy, but...
I have a fairly old ASUS notepad that was feeling slow and had pretty much run out of disc space. I replaced the old disc with a new 1TB Samsung EVO SSD, which has breathed a huge amount of new life into the old dog. (It was the notepad I was using when I bought BiaB 2013; dual-core Celeron IIRC).