This makes me sad. A product designed to be developer friendly and automate things should create a parser and custom config language, or at least borrow one. The whole reason there are so many is they are built for purpose to make a specific task easier. If you need features like templating and logic, you can slap in existing solutions like Jinja2 and Go templates. YAML is not a configuration format, it's a data serialization format. If you don't want to write a parser, use TOML.
My final note is: don't reinvent the wheel. There is a fully open source alternative to Drone.io (I can't remember the name) that forked a while ago. Drone is the best CI system for developers, hands down. Build on the open source codebase, build some proprietary extensions and a new UI, sell your product there. It's proven (because Harness now owns it) and you can help build the existing open source product at the same time.
Otherwise, I have no need for yet another build system. All build systems are pretty much the same to me (except Jenkins, which is pretty much the tenth circle of Hell) and I'd much rather use a fully open source one that I could pay for support + extensions for at work. Maybe you could make a better Jenkins?