I can't answer for what you should do, but the way you say "take advantage of" and "basically minimum wage" says you already are annoyed at the idea, and wouldn't be happy if you accepted with those terms.
There is risk with starting a new company - putting up the initial time and money - and work to be done sorting out accounts, marketing, sales, taking payments, legal concerns. Can you sort them out? Would you want to? Can your boss leverage the brand and customers of their existing company to jump-start this new business in a way you couldn't?
If you don't do it, what will your boss do? Would they easily find someone else or do you have particularly valuable knowledge of the industry/technology/idea, or a particularly good working relationship with your boss, or a track record of completing hard projects?
If you did it alone, do you know you can make money? What happens to you if you can't - how risky is it for your finances? Your family? Your career? Would it be a good skills building experience?
What happens if the new business fails? How much risk are you taking?
- Do you get paid anyway?
- For how many months, before the new business decides to close?
- Do you get your original job back? At the same Salary? Or better? (Will someone else have taken it?)
What happens if the new business succeeds and expands and needs more people?
- Would you build it for a time, go back to your original job, and new people would take over?
- Would you stay on working there, and have people hired to be above you?
- Would you expect to be promoted, and new people brought in to work in a team which you lead and train? Does your boss agree with you on this?
What happens during the initial build?
- Would your boss be micromanaging you? Leaving you alone? Calling you to ask you about your previous work and whether you can "just" do some more of it?
- Do you both have matching, clear ideas about exactly what you would be building, and how long it would take?
- How much does your boss understand software development and the time it takes, and how much do they trust your expertise on it? If they were expecting a result in two months and four months has gone by, what happens and can you deal with it?
My simple answer is that you shouldn't do it for just the same salary, especially if the salary is "basically minimum wage" and in the new position you would be designing, building, maintaining and taking responsibility for a new company's complete technology stack.
But then, if the plan is that your boss funds "more of the technology stack you use at your current company", that's less of a responsible position for you to be in, and so has less value.
If you are a 'minimum wage' employee, easily replaceable and with no bargaining power, it might boil down to "would you rather build new things from scratch for this salary or keep your current job at the same salary?".
What would you ideally want? Would you want to do it alone? Would you want part-ownership? Would you do it for a fixed salary - but a higher one? Would you do it for the same salary if you got a percentage of monthly profits after one year? Are you in a position to convince your boss to go with any of these?
Personally, I would want to work with someone else with proven experience at starting/running a business, so I could focus on the technology - but only if I was getting something I felt was a fair reward.