The better question is: do you want to go mainstream?

If your startup is satisfying a niche, then your strategy might be to jump to other niches. If you're solving the needs of grocery shops with a small ERP, then you might jump to bodegas, to bakeries, to small coffee shops.

Going "mainstream" is totally different - it's the final step, after you've conquered all the niches that you can think of, where the product is "generic" enough to spread to dozens or hundreds of tangential markets. Most niche startups never even make this transition.

The real question, therefore, is not if you should go mainstream. It's: is your niche market (and the subsequent niches you'll conquer) big enough and will it grow over time? it's that simple. There's no worry about building a product only for grocery shops if they pay enough and there is enough of them. If you're not making money off of them as the market isn't big enough, trying to generalize features and "phase-shift" to the mainstream will not only not work but make you lose focus and turn your product to crap.

You can make all kinds of market estimations, but at the end of the day, has the niche market (or markets) paid you enough to survive, and, even if not, do you truly feel that you can capture more value from them (by adding new features and cool stuff) in the near future? If yes, viable. If not, run away.

Take "mainstream" out of your dictionary and mind and focus on niche market size and growth.

I cringe at the cookie-cutter replies to this question. There is no right answer. It depends on the stage of your business and who the investor is.

If you're at seed stage and an individual angel investor is asking for 40%, sure, that is a deal breaker and you won't be "investible" afterwards as your cap table structure will be screwed. So that's a no, in principle. Even then, it's not a hard no. Do you really love your company and want to lead it during the next 10 years, or are you comfortable with answering to someone else and maybe even leaving after a few years? These are radically different situations. And even if you love your company, if you don't have a choice you might have to take the deal. But search for better terms from someone else first.

If, on the other hand, your company is bigger (Series B, Series C, etc) then it might be useful to bring in someone for a large chunk of equity - a venture capital, for example. Just be aware of the perils of this, which most founders aren't. Venture capitals are affected by whims (personal investor problems, need to give returns on a given year, and so many more random factors) and they will probably polarize your business model - they will try and force you to go for a billion-dollar exit with less chances than a reasonably profitable company - because their whole business model is based on that.

Give more info on the specific investors, your vertical, situation, financials and other terms you've obtained and I can give a more in-depth answer.

A good pitch deck is important for the serious entrepreneur. You and your team should focus enough time on your deck to ensure you have covered all essential areas:
- Problem + Market/Opportunity;
- Your Solution/Benefits/Product;
- Team + Advisory Board;
- Financials (Ask, Cash Burn, Runway, Funding Usage);

There is no telling how long you should invest in creating the pitch deck. When these essential areas are covered, you can consider your pitch deck complete. You should regularly update it based on product/financials changes and investor feedback.

And yes, you should have your pitch reviewed by an investor if there is one available. Be aware, though, that only investors that might realistically invest will give valuable feedback. If you ask feedback to an investor that is not in your area or is not interested in your project, he will give you "generic" feedback that might not be useful. What you want is specific tailored feedback from investors that truly match your company/product.