There are combinations of ATX motherboards and CPUs that idle at around 30 W.
However, the easiest way to have low idle power is to use a NUC-like computer.
Many models consume between 5 W and 10 W during light load at the wall plug, while those with the highest power consumption use between 10 W and 15 W at the wall plug, during light load.
The fastest SBCs with ARM cores that can be found at a reasonable price are comparable in speed with Intel Jasper Lake, but Raspberry Pi uses obsolete Cortex-A72 cores, which are 2 to 3 times slower.
Even a 7-year old NUC-like computer from Intel/Gigabyte/Zotac etc., with a "Core" CPU, e.g. a Skylake U, is much faster than anything with Jasper Lake or with Cortex-A76 or older ARM cores.
While a new NUC-like computer is more expensive, an old one might be as cheap as a Raspberry Pi, while being much faster.
During the last 20 years, I had a permanently active 24x7 server, which hosts a large number of services, e.g. firewall, router, NTP server, DNS server, DNS proxy and cache, e-mail server, Web server, Web proxy and cache, file server, etc.
The first server which did all that used the fastest Pentium 4 and it had an average power consumption of over 200 W. The current hardware of the server is the 5th version since the beginning. At each upgrade, the average power consumption has been cut into half. The current server consumes around 10 W and it uses an Intel NUC with a Coffee Lake U CPU, together with 4 USB to Ethernet adapters, to provide 5 Ethernet ports.
I have experimented with various ARM-based SBCs, but when old NUCs are available, repurposing them is a higher-performance solution.