How do I plan dinners that still happen on the bad days?
Plan four, not seven, and make one of them deliberately trivial. The plans that fail are the ones with no slack, because a single late evening cascades into ordering food for the rest of the week.
The rear derailleur skips on the third gear when I push hard.
Cable tension first: a quarter turn on the barrel adjuster while pedalling usually finds it. If it only skips under load and shifts cleanly otherwise, suspect a worn cassette paired with a new chain, which no amount of adjustment fixes.
Which mini PC is quiet enough to sit in a bedroom?
Anything fanless with a passive case, or a laptop-class chip undervolted and capped below its turbo. The noise people actually notice is not the fan at load but the pulsing at idle, so a fan curve with a wide hysteresis matters more than the rated decibels.
How do I know when it stops being worth it?
When you stop noticing it. If it has not surfaced a problem in six months, it is either solved or it was never the bottleneck you thought.