That is what I was doing (and still do). I wanted to learn how to do it on a smaller form factor device. For example, imagine a firewall in the form factor of a smartphone. I have seen at least one other person on HN who says they are using a pocket-sized gateway/firewall.
Intentional "MITM" is obviously a common topic that gets discussed on HN from time to time. What bothers me about HN commenters on this topic is that they signal that they are familiar with what needs to be done, i.e., to MITM their own traffic, and want us to know they understand how one "could" do it, however it is unclear that they themselves are actually doing it. I sometimes wonder if this is tacit approval of TLS being used (strategically perhaps) to lock users out of monitoring their own computers and computer networks. Hopefully there are many HN readers who do monitor ther own traffic but are generally not commenting about it.