Seven years of agency experience is a solid foundation for moving in-house, but I suspect the way you're framing it in interviews might be selling you short.
Most in-house hiring managers want someone who can own a channel and deliver execution from day one. If you lead with "I want to collaborate cross-functionally and understand lead quality," they hear "I want to be a strategist straight away" - and they'll pick a candidate with narrower but deeper in-house experience instead.
The HubSpot / Salesforce gap isn't the real blocker. It's a signal issue, not a skills one. grab the HubSpot certification over a weekend, put it on your profile, and move on. Problem solved.
What actually gets you through the door is positioning your agency background as a strength. You've seen how a dozen different companies run their marketing, you know what works across industries, and you've already done the exact job they're hiring for. Most in-house candidates have only ever seen one company's way of doing things.
Also, target growth-stage companies - Series A to C, or bootstrapped but scaling. They need someone who can operate independently, and that's exactly what agency life trains you to do.
The internship idea? Drop it. it won't help and it signals desperation. Focus on targeted applications with a sharp, specific pitch instead of volume.