This is a common misunderstanding of Descartes. If you actually read and contemplate Descartes Meditations you'll see that the argument is not, "I think, therefore I am". The argument is: if thoughts or any perceptions are arising at all, they must be arising for me. Which means I must exist even if the thoughts and perceptions are a deception. The one thing which cannot be a deception is the I AM. Even if I think I am not, I still AM.
Descartes was not saying that thinking causes I AMNESS. He was saying that thinking arises within I AMNESS. There cannot be thoughts without I AMNESS.

I have an automated system that backs up all my computers daily to a local 40TB NAS storage unit which has a raid array.
You can buy small automated external drives which connect to your local network. Do some research. It's pretty easy to set up these days.
Or if you are cheap and lazy, cloud.

https://www.actualized.org/insights/profound-quotes-016
That is the paradox of life. All meaning is imaginary, so what kind of meaning do you want to imagine? If you don't imagine consciously, you will imagine unconsciously. Not imagining is not an option. You are going to imagine something. You are going to create something. So what do you want to create?
Life purpose and detachment are not opposites, they go hand in hand.

The book doesn't instruct you to do all these things simultaneously. You precede the exercise with Nadhi Sodhana Pranayama/ Ujjayi Pranayama (which you would be doing anyway if you are following the book properly). Then I believe the text is saying focus on Brumadya as another preliminary exercise, although after several weeks skip this step (the wording is a little ambiguous here, but I'm pretty sure I'm right). After these steps you move onto the the chakra visualisation/ om chanting, which includes focusing on medulla at the end of the sequence, as you would the first 5 chakras (i.e you shouldn't be concentrating on this point all the way through). In other words, there is only ever one point of focus. Hope this is helpful.