The amount products has no effect on which theme you choose, other than design (like if you have 1 product you would choose a theme geared towards a single product).
I vote for any theme by Out of the Sandbox, they’ve been my go to company for site build projects for about 6 years now. As a developer myself I appreciate their attention to detail on putting out quality code.
Haha ya I’ve heard this from a lot of clients about their support being bad, I’m not sure why they don’t have better support but their support quality does not reflect their developer quality.
If you’re paying under $500 for a professional theme with lots of features you’re getting a smoking deal. Most sites I’ve worked with stay with the same theme for 2 - 5 years, so consider it a long term investment.
As for performance, if you’re buying a modern, professional (paid) theme, then the performance difference is usually quite small. Most performance issues happen after customizations and apps. As long as the theme has lazy loading and good control of image sizing (which most pro themes have now), then you should be good. There are other tactics for speed that some themes have and some don’t (like preloading, which Turbo utilizes to some extent), but most performance can be dialed in with any theme.
For reference on my suggestions, my team speedboostr.com have expertise in Shopify performance optimization. We’ve optimized 1,000+ sites and built the first performance analysis tool specifically for Shopify: the Shopify Analyzer… free for the community to use. Whatever theme you get, you can run some pages through there to see where it stands, with a new theme at most you’ll only need to spend a few hundred on optimization, but most likely you want need to spend anything.
Turbo has 1 fault that I’ll point out, their lazy loading is not true lazy loading (they do placeholder lazy loading), so you’ll see warnings pop on analysis tools. That’s a small project to fix if you want to have it implemented… or you can reference this guide: Lazy Loading on Shopify and replace your src attributes on images with data-src (your developer will know what that means 
).
Good luck with the new project!