Once you eliminate the washer-based leveling system, the differences between the Taz 5 and 6 are more or less just improvements to the frame and toolhead. BLTouch firmware for a 6 works for a 5.
I’m not sure if the linked firmware in that document will even compile anymore, as it’s Marlin 1.8.x, so dependencies are likely to be broken. Even compiling some older versions of 2.x are problematic these days.
Physical install is about the same as in the document. It varies based on what toolhead you’re using, but there are several pre-built toolhead files you can find on thingiverse or printables.
As for a more modern install guide, it’s covered it fairly well in this thread here: Stock Taz 6 BL Touch add on - #3 by Samuel