The IRS wants to encourage productive economic activity. Letting businesses deduct expenses can mean that later they end up employing people who buy stuff and themselves pay taxes.
Also cottage industries that barely pay for themselves are very inefficient and governments usually want to discourage them so you'll do something more economically productive with your time.
Yeah, it makes me wonder if OP would actually enjoy the new position if they got it, considering that would be the job. There are definitely people that enjoy that kind of work. So yeah, definitely don't write that on the application.
Let's Encrypt is just as secure as paid certs. They're held to the same security standard.