Correct. Micro-ATX is the smaller version of the larger ATX and still larger EATX (extended atx). Your old case probably fits micro atx if it’s not OEM. You can populate it with a mb, cpu, ram, ssd, and power supply (don’t need more than 500w for your use case) and eventually move to a nicer case like that Node if/when you fall in love with the hobby. My Rpis are collecting dust since switching to a low power server.
It’s a whole different experience when general advice applies to your hardware vs the Rpi ecosystem. Many more options. In 2024, ATX offers no real benefit over the smaller form factor beyond better heat management for high power builds with spaced out components.
And a correction: node 304 supports 6HD, the 804 supports 8