I've used Instantly and a couple of other sending platforms for cold email. The issue with Instantly is that it's $100/mo + G Suite domains. And I don't constantly cold email, so it gets expensive in the months I'm not using.

I find Reach Inbox to be better than Instantly, user support is better, oauth with G-suite is amazing and easy. I love this app, it's my best Appsumo purchase x10.