Don’t use generic phrases, since the hiring managers see hundreds of motivation letters every day! You need to grab the attention of the reader, therefore we gave you rather unconventional examples, so you can stick out from the immense crowd of cover letters.
Don’t use abbreviations like “I’m” or “I’d”, because with formal English writing, you should use “I am” and “I would”.
For startups, you don’t have to be super conventional and start every sentence with an extremely formal phrase. Find the fine balance of staying formal but also delivering a bit of your personality, giving an added value to your letter of motivation.