I had the exact same experience last year and it fucked me up good with anxiety (which I've never experienced before) and I couldn't eat properly for weeks causing me to lose a stone in weight.
I had the exact same preconceived notions you did with the 1 hour block a day mon-friday. Seemed like it would be a breeze at first and easy money. The problem with this is it just turned into feeling guilty when I wasn't working on it, even outside of the allotted hour. I think I felt like this because there was no clear seperation of environment as I was be doing the work at home and the result was I felt like I was just working all the time.
I did learn a valuable lesson though and that is that my free time is a lot more valuable than I realise.
I will never again consider doing side gigs for anyone other than myself.
I was under the impression that I would be able to do it because back when I was in high school and college I can easily get good grades with barely any effort. I even had time to enjoy other leisure activities. Turns out, I was just oblivious what real responsibilities are.
One issue in particular that I ran into was the difficulty in context switching between my day job and my side job. I imagined being able to do an hour here or there before or after work. But in reality I found it really hard to get any meaningful work done in an hour, which meant I needed to allocate larger chunks of time than I originally planned. That meant I often ended up sacrificing entire evenings, or chunks of my weekend on my side job. The difficulty in context switching also caused me to procrastinate a lot, which also made me feel terrible about myself.