Process evolution at startup is always a challenge.
Your current team (one developer and 2 advisors) is in actual one member team ( I am assuming that advisors are not doing any technical software development)
Formal SDLC approach will be a challenge and it will always be for this software developer to juggle
a) New feature / functionality development
b) Defect resolution from past year release
c) Any incident , defect or change in the past year release
My suggestion would be to build an ServiceDesk like item log ( divided into three categories above) and prioritize for the developer in consultation with manager and technical advisor.