I for e.g. already wish I had spent a "bit" less time on ideating and a bit more time on getting in to flutter (as now I've gone in to an iOS mode and I know that's not scalable at the start since we won't have man power to maintain an android and ios codebase)
Don't run all network services on a Gentoo box.
Don't write yet another C++ strings library.
Don't write yet another C library. (that is, a libc implementation)
Don't put developer workstations on a network that connects to the internet.