Dedicated video camera?

I currently have two devices for video. My iPhone 8 that shoots 4K at 60 FPS and slo-mo at 1080 at 240 FPS. I also have a Nikon D3300 DSLR that can shoot 1080 at 60 FPS. I have 18-55 and 50-200 kit lenses.   Use would be general purpose home video, maybe go as far as getting on YouTube. My concerns with my current devices is optical zoom(iPhone), audio quality(both), and ease of use(DSLR).  

How much camera would I need to be ahead of what I have now, and what would it cost? Otherwise, do I just make the best of what I have as, 1, with my phone, the best (applicable device) is the one you have with you and, 2, with my camera add an external mic(it has the input for one). 

Bonus, everyone needs a GoPro, where would that fall in this discussion?

In reply to Brian :

Definitely depends on the quality you are going for.

I have 2 GoPro Hero 3+ Silvers, and I use them for in car video and action video. They work well for that, but I tried to record a play onstage and some of the low lighting and lots of lighting made it not work well for that at all. A newer GoPro (5 or newer I think is best right now IIRC) may do better. No zooming on a GoPro, so that may be a downside for you. My generation at least does have external mic input options. Built in audio is ok depending on which case you have it in, not great, but not horrible. You can do a lot with them though, Streetspeed717 on YouTube does all of his videos just with a GoPro and the quality is decent enough for that. 

To get any higher quality you'd definitely want a "real" video camera. I've seen some people use DSLRs with mics and stuff too, so that might be an option, I really can't give too much input there. 

Most likely the biggest thing that will make your videos way nicer is if you're not using a tripod, get some sort of gimbal. They use gravity and/or motors to move the camera to keep the video smooth. Now, personally I have not used one, but they do seem to add a level of quality for any recordings that the camera is moving during. A newer GoPro and a small gimbal can do a lot. 

I like my DSLR for video, but I don't do a ton of it.  The quality tends to be pretty good with the kit lenses and the prime lens takes it up a further notch.  It's easy to mount on a real tripod so I can get good steady footage.  Downsides are that the audio isn't great, it's kind of delicate and expensive, and the weight and bulk negates a lot of smaller tripods or mounts.  If I'm doing an instructional video of some sort, I'll blend footage using the DSLR for the "wide shots" or a close-up that I want the best quality on and then GoPro cameras mounted to my body somewhere or on a clip mount for the "action shot".  I've even used a tiny little digital camera in its video mode to get into some tighter shots when the GoPros weren't handy and that doesn't turn out bad for YouTube.  

is there a good USED  video camera that was state of the art a couple years ago and is now cheap USED on Ebay etc ?

and how fast of an SD card do you need ?   I know the cheaper SD cards will not work very well.

Thanks for the info