Oh honey, 5 replies out of 1,000 sends isn't a deliverability issue, it's a "nobody gives a toss about your email" issue. That 0.5% reply rate means your copy and your list are fighting each other, and both are losing.
"Free video ideas" - mate, every content agency on the planet is peddling that. It's not a value proposition, it's bait that looks exactly like the other 50 emails in someone's spam folder. Reframe around a specific business outcome, not a deliverable. "We'll help you close 3 more deals this quarter" sounds a lot less like a template.
And while you're at it, check your ICP. Content agencies love to cast a wide net and end up emailing companies that wouldn't know a content gap if it fell on them. Tighten your targeting - go after businesses with obvious content holes, and watch your reply rates climb way faster than any copy rewrite will.