The easiest option is to get a full-time job at some place that needs the type of OSS code you write and negotiate your employment contract to specify exactly w

The easiest option is to get a full-time job at some place that needs the type of OSS code you write and negotiate your employment contract to specify exactly what will stay yours and/or open. In terms of sustaining a living, the second best option is IMHO dual-licensing GPL/AGPL and a commercial license. However, note that this isn't really writing FOSS for a living. This is mostly a job of making sales calls, reviewing contract changes, and chasing invoices.