Is it easy to write a 30-page e-book in a weekend that’s a bestseller?

It is next to impossible. Best-seller status (at least if you define it in the way big publishers do) is only ever there for novels and non-fiction of book length, which would be considerably longer than 30 pages. As an eBook only format you further reduce your chances.

There are exceptions.

• Minecraft, Raspberry Pi and Video Game Guides. If you catch the crest of a huge wave and get a really great book out when something HUGE is launching, you might get a bestseller. Many Publishers were astonished that Minecraft topped the charts last year, outselling everything else.
• Childrens picture books. You’d be struggling to do art and text in one weekend, but at least the lower page count would work for you, not against. As for eBook, again, no. It has to be print.
• Someone important dies, and leaves no biographer. It’s unlikely, but again in this case you could steal a few weeks of sales before the official book comes out.

Aiming for Amazon category best-seller is another matter, and this is where I would recommend expending your efforts. If you find a niche category you may be able to be the best-seller in there.