Best Cold Email Software for B2B SaaS?

hey I have an app, I want to market it with cold email.

I was initally gonna use instantly but they have a small email list of only 200k in total so less than 2k of emails of bosses using shopify. What else should I use or should I just import my own email list into instantly?

Also I am using the cold email method where I have 7 domains, and 3 business emails for each of these, meaning 21 total emails, and I am told I should send no more than 50 mails a day per email. What do you think of this stategy?, Is it correct as I got it from the instantly youtube channel so I wanted to know if I was getting scammed or.

Any recommendations or help on this domain would be very usefull thanks.

My app is a shopify app called PlusSales btw