My 15 y/o granddaughter produces short videos using an IOS-only workflow that is probably exactly what you are asking for. Her work is of professional quality, and she has become an Instagram micro-influencer with 34,000 or so followers.
She shoots the video on a Sony a6400 camera and sends the video files wirelessly to an iPad Pro. All video editing (trimming, clipping, transitions, sound, etc.) is done in the iPadOS app Luma Fusion, then uploaded to Instagram. I’m not sure if a separate transcoding step is required but if so, Luma Fusion does it.
She switched to this new workflow about 6 weeks ago after reaching the limits of what the iPhone and iMovie could produce. She initially asked me to help her obtain a MacBook and decide on a video editing app “because that’s what the Pros used”. After a bit of research, I discovered that a number of Professional video producers have discovered that Luma Fusion on an iPad Pro is fantastic and has advantages over a Mac-based (or PC-based) workflow in terms of production efficiency. I bought her the iPad Pro and introduced her to Luma Fusion. The rest was history … she took to this like a duck to water. She does amazing work and now produces more videos than ever, at least one fully-edited and polished Instagram video per day. She would not even consider using a Mac or PC now that she has experience with the iPad.
For what you are describing, I highly recommend that you check out Luma Fusion and iPad video production. No Mac or PC is required. You could continue to use the iPhone for the video camera, but you may want to look at a separate video camera for additional capabilities such as an external shotgun or lavalier microphone. I recommend an iPad Pro with a large memory capacity (perhaps 512 GB). When you eventually run out of memory, it is easy to connect an external SSD to the iPad Pro for storing and archiving your work.
Based on what my granddaughter has accomplished in a few short weeks and what other video pros are doing, it appears that we are at an inflection point. More and more video processing will be done on iPads. Today’s teenagers are quite at home with touchscreen devices. They don’t need or want a computer, keyboard and mouse to produce video.
FYI, below is a link to a short youtube video in which a professional photographer describes video processing on an iPad Pro using Luma Fusion. Since this youtube video was published, version 2 of Luma Fusion has been released with additional capabilities.