My favorite design idea: move your computer out of the room. My PC is in the next room / closet and the room is completely silent. In my case all I needed was a small hole for an HDMI and a USB cable.
If you don't need a high-end GPU, it's pretty easy to make a PC essentially silent. And even for a GPU it's possible if you're willing to sacrifice some performance and lower the power target. My desktop is not specifically designed to be silent, but the CPU and case fans are not audible from a typical distance.
Honestly I find people claiming something is "inaudible" or "not audible" on the internet extremely dubious, because every time I buy some cooler that people claim is "inaudible", I find it very audible. Either I have good ears, or people have very bad ears or loud environments. I assume in many of these cases people simply live in loud cities and have lost any feeling for what the word quiet means and regard PC fans whirling around at 20-25 dBA as quiet.
And in any case, with air cooling and components that aren't bottom-of-the-barrel, the quiet part ceases as soon as load is applied anyway. But that's what I'm here for, so a PC that's only quiet as long as I don't use it - pointless.