How to survey users about their level of engagement?

So today I did soft launch of my SaaS tool. I sent out about 150 emails to my mailing list. Roughly 45% of people opened it. 20% of users clicked it and landed on my site.

I found that I had only 20 people sign up for membership but only 4 bothered to engage with the product. However, even the engagement was short lasting, after they created some jobs, they stopped.

What is my next move here? Do I email them and ask hey what didn't you sign up? What about the other people? Did you find the website poor if so how? What did you not create more jobs? Basically I don't know what I'm doing wrong and what I'm doing right is making me nervous.

What you describe is a client acquisition funnel. You need to define personas, aka your potential customer and how you can reach for each of them at every different funnel step, in order to increase your potential success.

User that signed up you can potentially directly communicate with them. Users who did open the tool but didn't signed up you could send them some more promotional offers. People who did opened the email but didn't click on the link possibly didn't had an interest in doing so, so you must provide more insight on your tool to spark such interest. People who didn't opened the email have either been turned down by your mail title or just didn't bothered.

Dont discourage yourself, starting a SaaS business takes time and is a matter of spreading the word as much as possible. Try finding the best way to publicize your tool to your intended audience.