Is 0patch FREE a suitable tool for keeping a Windows system reasonably secure after EOS?

No. 0patch FREE is suitable for non-work-related computers (see this article) that are still being updated with original vendor's updates/patches, and is meant as a free additional layer of defense on top of vendor updates. 0patch FREE includes only "0day" patches, i.e., patches for vulnerabilities that original vendors haven't fixed yet. As soon as the original vendor's fix becomes available, such patches are no longer included in 0patch FREE.

Therefore, 0patch FREE is by no means a suitable method for keeping Windows Workstations and Servers reasonably secure after their EOS. Patches for Windows systems that have gone out of support are only available with 0patch PRO or Enterprise license.