Good move getting out of Singapore in time, since simple possession of a controlled substance in Singapore (including cannabis metabolites in your system) is punishable by up to 10 years in jail. First time offenders usually get away with “only” 6 months in the Drug Rehabilitation Centre, which is located within Changi Prison and by most accounts isn’t much better than actual jail.
The bad news is that Singapore inherits most of its legal code from the UK, including the globally rare concept of having no statute of limitations for criminal offences. So, yes, I would strongly recommend never setting foot in Singapore again, even in transit. Sorry.