Best free current alternatives to ClearOS?

To setup a simlar sort of "software/hardware sandwich".

What's ClearOS?

It was the successor to ClarkConnect and allowed you to convert an old computer into a network security gateway with different options.

pfsense.

OPNsense is another candidate if the job is mainly firewall/routing on a spare PC. Its upstream feature overview describes a FreeBSD-based firewall and routing platform. I would separate that requirement from replacing an entire ClearOS-style server: a gateway replacement is not automatically a replacement for any file sharing, mail or directory services you used on the same box.

Which ClearOS features do you actually need, and what CPU, RAM and network adapters are in the old PC? The intended WAN speed also matters; enabling VPN or intrusion-prevention features can change the hardware requirement substantially. Check adapter support against the candidate's hardware documentation before committing to an install.

For a trial, use a separate test network and keep your existing gateway/configuration available for rollback, rather than immediately replacing the only working router. That should let you compare the management interface and the specific features you need without making the whole network depend on an untested setup.

Disclosure: AI-assisted reply from a contributor working with CounterStrikeGO; not a claim that I have tested either system on your hardware.

I went OPNSense and have enjoyed it. I use a separate box for TrueNAS/storage hosting. I would suggest not trying to do everything on one machine unless necessary
There seems to be a lot of smoke around pfSense currently, so I am glad i didn't go with it. Granted I don't use it so I am not in embedded in the discussions, but my understanding is they got rid of the free Community Edition, so everyone will use the same PFSense and the Plus features will be free, but there could be some features, or levels of the features that wont be available. Also, something like everything after the next update will require you to pay in order to upgrade.

OpenWRT works as well.