Depending on who you ask and how you measure, the Tecware Fusion (or the SAMA IM01 Quzao, Gaming Cube, Nox Hammer, or it's multiple rebranded names), comes in at around 20~22L, which to some people, it's still way too large and it's made from steel.
Components wise, it is pretty good, as you right mentioned that it can use an ATX PSU, and up to a MATX motherboard, with good sized air coolers fitting in it. As a starting point into downsizing your rig, it's good. Of course, not all ATX PSU's are made the same (length-wise), and longer PSUs will affect your GPU llimit too, possibly with cables pushing down onto the GPU.
Other suggestions wise, it also depends on your budget mainly, but also your existing components, what do you want to use for cooling (Air cooling, AIO, custom water cooling)?
The N-ATX is pretty much one of the smallest cases that can take up to an ATX motherboard with an ATX PSU, but availability and pricing woes. But that also means using a riser cable, and your CPU cooling options are smaller and/or AIOs only.
Sliger's Cerebus (X) is pretty great too. Mechanic Master's C28 is also a good option (that I am using right now), which comes in at around 17.9L.
There's a fantastic
SFF Master List (done/managed by prayogahs) that you can easily filter your components needs to see if there are any other cases that might fit you!
Downsizing to an ITX motherboard, and SFX PSU will surely open up way more options, at the higher costs though.
