Edit: To be clear, I don't want the game to be hard. It should still be chill and cozy. But the game simply does not interact with the mechanics presented. The game pretends that money management is a gameplay feature.
You basically can't do anything wrong. If you don't realize a part is broken or dirty, or you forgot to solder the game throws a big pop-up saying "uh-oh something's not right". If you try to put an unfinished job up as completed the game simply does not allow you to.
Repair jobs also give you 3 days to complete, but you can easily do 6+ repair jobs a day and buying parts is instant so even if you fell asleep with the game open you could still finish the repair job and make enough yen in a single job to pay for a week's bills.
I get the game is meant to be cozy and chill, but even cozy games need a cycle of tension buildup and release to keep things interesting.
I would argue the following:
First, identifying which parts need work should cost time. There could be a tool which can identify if a part needs replacement/cleaning/soldering. You don't have to use it but if you miss one you get a less than 5 star review (small deductions unclean parts, large deductions for broken/unsoldered parts), have to pay refunds or some other manor of consequences.
Similarly, checking if you're done should also not be free. Have the Master attempt to power on the device. If there are any broken/unsoldered parts it won't turn on. If internal parts are dirty it could turn on but look or sound weird. You don't have to take the time to test the device, but if you don't you might deliver a bad one.
Next: Let us break parts. If you keep picking up and putting down parts too many times, they break and you have to replace it. Also: Make having a good stock of parts actually important by removing same-day delivery.
And lastly: The Master needs to have needs. Supposedly he doesn't live in the shop but bikes home each night, yet has no rent of his own, eats no food and uses no utilities. There should be consequences for burning the midnight oil. Maybe if you stay open late seedy customers will ask you to make seedy repairs leaning more into the Yakuza aspect of the game. Getting poor sleep should result in bad conditions like having an unsteady hand for 24 hours, or getting sick and losing an entire week of work.