"Building is cheap. Owning isn't" is the right way to frame this now. The question used to be whether you had time to build something, and now it is who looks after it once the AI has written it.
Question four is the one I would underline. A broken settings page gets reported the same day, while a broken notification email can fail quietly for weeks. Anything that runs when nobody is looking needs monitoring, and that cost is invisible on the day the feature is generated.
The feature request board is a fair test because the visible part is a list and the hard parts are vote limits, spam and moderation. That pattern of a small visible surface over a large invisible one shows up in auth, payments and email too.
I also appreciated you saying up front that you sell one of the options. At Ascendra Ventures we use a similar test when a client asks for a custom build, and sometimes the honest answer is to install a service and spend the budget elsewhere. Did the Lovable build handle duplicate votes at all without being asked?