I strongly recommend reading more on the subject and carefully following a known-good example configuration.
I recall that you need to enable 'pf' and add RDR rules if you want to use Squid as a 'transparent' proxy.
You can use Squid as an 'explicit' proxy (by putting proxy settings in the browser, or using WPAD), with or without enabling pf and ip forwarding.
BTW, looks like a typo, the line in sysctl.conf should be "net.inet.ip.forwarding=1"