Successful and straightforward entrepreneurship. Buying well-positioned property outright for a reasonable cost (with shockingly low taxes) and having the resources and backing to create a business, hire good employees, and be basically guaranteed to corner the market and garner goodwill with your innovations. The cleaner slimes naturally maintain themselves, reproduce in proportion to the amount they eat, and require no pay. The entire store can be managed by one person comfortable with slimes, and the overhead cost is entirely in buying the land and laundry bags and paying their limited upkeep. He can go adventure with basically no loss of business prospects, and has close ties now with every major figure in local business and regional politics, as well as the goodwill of the community he plans to provide services to. Literally the only meaningful downside is that he has to learn basic management skills and to delegate responsibilities, which is something that will end up being incredibly useful to him anyways. He even has 60-70 years of life ahead of him as a child to grow the business and accumulate wealth via reinvestment and diversification of his business into new slime-based ventures and properties.