I have a bunch of cameras and video cameras and have changed them a lot through the years. I like photography and I enjoyed having all the equipment to record things like my kids' sports, school events, life events, etc - so I had use-cases beyond rocketry that allowed me to justify (at least in my mind
) buying higher end stuff. So, I have a fair amount of experience trying to get good video of rockets.
A bit of a bottom line up front - for flights over about 2000 feet, you are going to have trouble with any cell phone based setup. By the time you spend the money on cell phone enhancements that would get you in the ballpark of decent video, you may as well have bought something dedicated.
For 35mm cameras, I have been a Canon guy since the 1970's, so I don't have a lot of Nikon experience. For about the past 15 years, my main shooter was some version of an EOS 5D and I also had a number of APS-C cameras. Over the past few decades, I accumulated a lot of excellent EF and EF-S lenses (both Canon and 3rd party). Last year, I finally bought a mirrorless camera - the Canon R6 Mark II. I am still learning to shoot with this new tech. The change from SLR to mirrorless is tangible. Lots of great things I love, lots of things I miss. In retrospect, I do wish I had spent the extra for an R5 as the R6 is not the same as a 5D, but it is still a great camera.
35mm cameras are amazing for still photos of rockets. There is nothing else that compares. You can get amazing stills from phones, but even a cheap 35mm will likely beat it. Mainly that is because of the opportunity to buy lenses purpose built for fast motion photography.
Using a 35mm for video - especially fast motion video - is not as easy. The more high end the camera is, the more manual it is. Pros don't get good stills/video from high end cameras because it is easy, they get good results because they spend huge amounts of hours training on their skills and huge amounts of money to augment their setups. As you move down the ladder of 35mm cameras, the ease of use increases, but the capabilities of the camera decrease rapidly. The best analogy I can think of is: Buying a high end 35mm and only using it once in a while is like trying to commute to work in a Formula 1 racecar a few times a year. It only sounds fun until you try it.
Here is the TL;DR - if your goal is video of rockets, I recommend an "old school" solution - get a dedicated camcorder. I have a couple and I love them for this purpose. The consumer/prosumer camcorder industry is not gone yet, but, unfortunately, it is rapidly disappearing due to ubiquitous cell phone video. Luckily, there are still good options, but, when they are finally end-of-life, I have a feeling manufacturers don't have anything new in the pipeline, so I would grab what you can now. The main price differentiation is resolution. If you want 4K video and anything remotely approaching a good camcorder, you are going to spend $700+. If you want a 4K that is good/really good, $1000+ is the starting point. The good news is that, if you can live with 1080p, you can get some amazing camcorders with excellent zoom for about $300+.
I have found that the best way to get video of your rocket launches is some combination of all of the above. For a launch I really want to capture, I have Runcam 2 4Ks onboard (one pointed up and one pointed down), I have a couple/few gopros around the pad, I have a 35mm on a tripod to capture stills, and I handhold a camcorder to catch the flight. Pro-tip - don't use the screen to track your flight. Get an eye cup for the viewfinder and use it. It is really difficult to see the screen on a sunny day and track your rocket while zooming, moving, etc. It is way easier to do all that through the view finder.
This isn't my best work, but shows a bit of how you can track with a camcorder:
I have posted these 2 videos WAY too much, but they do highlight what you can do with a variety of cameras:
Word of caution - the rocketry hobby is addictive and expensive, but it absolutely pales compared to the deep dark hole you can fall down once you start accumulating photo/video gear