Other option for Chrome OS Flex exists.
Other option for Chrome OS Flex exists.
You're right about the limitations of a single-core CPU with modern operating systems. It seems a lightweight Linux distribution could work better. Debian paired with XFCE4 might function, and Puppy Linux could also be a viable option—though it hasn't been used much recently. RAM availability is crucial; if it's limited, you'll likely face slower performance and possible disk swapping.
You could simply install Ubuntu and use a simple desktop like MATE or LXDE. Those older Atom processors are pretty slow, but the CPU itself isn’t terrible. What are your plans for this machine? Would buying a used Sandy Bridge or newer laptop or office PC be a good choice for you?