Hi, I'm the community engagement manager @ Android. It's my understanding that you don't have to keep developer options enabled after you enable the advanced flow. Once you make the change on your device, it's enabled.
If you turn off developer options, then to turn off the advanced flow, you would first have to turn developer options back on.
>- One-day (day!!!) waiting period to activate (one-time) -- the vast majority of people who need to sideload something will probably not be willing to wait a day, and will thus just not sideload unless they really have no choice for what they need.
ADB installs are not impacted by the waiting period, so that is an option if you need to install certain unregistered applications immediately.