TL: DR I've been freelancing for 4 years now, started from scratch and learned everything over the internet. Most of my high-paying clients, I closed them via email only.
But, most of the cold emails I sent never opened. They were a sunk cost. Until I hit a sweet spot strategy that kickstarted the cold email closing rate.
Here are the three things I've learned:
A short story about the importance of research: After struggling to get a sales job, Justin Reidt applied to join a company. He found that his interviewer (Sean Gentry) is a Lakers Fan, so he changed his Zoom background to a photo of Staples Center (where Lakers play).
The result? Sean got surprised and immediately hooked to what Justin has to say. He even mentioned this story on LinkedIn.

Well played, Justin. Really, well played.
Aim for personalization in emails too. Research about the recipient and get the idea of his interests. Then, tailor the email around his interests him in a natural way. And you'll stand out from the crowd.
You have only 10 seconds to win the attention and interest. Make it count.
PS: Justin got the job just after the interview.
Which one is more likely to get a reply? The second one, right?

Cognitive bandwidth is our cognitive capacity and ability to pay attention, make good decisions, and stick with our plans. It's a thinking bar.
To grab attention, short circuit the decision-making of the recipient. Don't let him consume his time to make decisions. Make decisions for them so that all they have to do is hit send.
Example: "What about a call at 4 PM IST on 24-06-2021?" Or "I have picked [date and time] from you Calendly, can we meet?"
This is an extension of the previous point. Present all the resources within the email itself. Cut down every unnecessary step he has to perform to get the information. Make it readily available.
Instead of different links to portfolio, website, LinkedIn, share a Notion page highlighting every crucial information needed. Put a word limit and write for skimming.
Remember: Emails are the new postcards—for a human, from a human.