So it is kinda unavoidable unless you cheat the money? Even with under 10 days there will be some scenes with the rival, so yeah it's kind of...
Even if you cheat it, you'll have to hit your goal within a day or so to avoid anything happening, as the first event triggers within the week. Obviously this is designed to be impossible and even if do cheat the goal before the rival makes any moves, it will still make the so called "Happy/Normal Ending" come off weird as the text will still imply that something happened.
It's possible btw to reach the goal without any cheat cheeses but I can't help but feel that this ending isn't canon from the dev's respective, hence it's made challenging. The idea is to balance your expenses and income. Here are some tips:
- While it might seem good to equip yourself to the teeth, it will also push your goal farther out. Only get what you absolutely need.
- Offence is better than defence. Focus on getting a weapon that ends the battle quickly.
- Understand that the mobs drop exponentially better stuff as you delve deeper. Don't overreach, but don't spend your limited battle counts on trash either.
- Your sister is a one shot power house. When with her, aim for the top floors i.e the ogres. They will give you exponentially more valuable drops.
- Mobs can drop rare stuff, but they also cost more to open. And can contain trash as well. Grinding might be necessary including save scumming for better RNG/evasion.
- Make sure to hit the quest from the student immediately. It will give you a sword. But most importantly, if you miss it that day, it's gone.
- The limited items in the shop are an illusion. They are available all the time. You just need to cycle through them by talking to the NPC until you see what you want. The sword especially is overpowered though might not be something you want to pick due to cost.
- Sell the orb that your sister offers you after you finish her personal quest. It will help you get to the goal.
If you follow this logic, then any NTR is unavoidable because a rival approaches mc's girlfriend.
Which is supposedly the dev's intent. Hence the moronic thought pattern design of the main character. He knows he doesn't have the skill to fight his rival. And yet, bites the obvious bait anyway and gets his ass handed to him, pretty much locking the heroine into the NTR corner.