Just don't buy DJI -- they absolutely want lock you in to their tools, parts are often not compatible even within DJI, require to create an online account, require an app (from a custom .apk on android) and in general have questionable privacy.
Of the open-source systems there's a new OpenIPC system with a most popular implementation of RunCam WifiLink 2 that supports onboard SD card recording [1] [2].
More proprietary (but still cross-compatible with others) is Walksnail Avatar V2 [3] with 32GB of internal storage.
For your case, you don't need a VRX (receiver), although you can totally give it your your buddies to see your race (with OSD) in real time. VRX can be built-in to goggles (if the same company), or as a separate module that connects to your preferred goggles over mini-HDMI, also with recording. [4] [5]
[1] https://www.youtube.com/watch?v=BP7Ns7H9wvI&t=49s
[2] https://shop.runcam.com/action-camera-categorie/
[3] https://www.caddxfpv.com/products/walksnail-avatar-hd-kit-v2
[4] https://shop.runcam.com/runcam-wifilink-rx/
[5] https://www.caddxfpv.com/products/walksnail-avatar-fpv-vrx-o...
Of courses, for more advanced stuff you might want to provide the telemetry yourself (like the gopro doesn't know your wattage). But it does have much more than dji out of the box.
Been a while since I used it, but it will generate the overlays and you can sync it with your ride data (eg Strava or Apple Health in my case, but iirc it also supports Garmin Connect).
There are some capability differences between the mobile app and the Insta360 studio desktop app.
I'm pretty sure it handled multiple files, but in my case they were the chunks that the camera splits its recording into, which is a bit different than than having multiple clips as you described.
Judging by the paucity of software to do this, historically, it is not a straightforward problem, or all the devices involved don't generate all the data points required.
The real mess is when you have 26 clips from a long event to string together. It can easily take a day and a half to make a 3 minute montage out of that.
Just prompting claude (probably I would start with Opus) "I would like a HUD display of the following metrics from my Fit file overlayed on these GoPro videos, and I'd like the videos stitched together (there are some gaps, I want seamless playback) it would probably do it in 30 minutes or less, and the majority of the time would probably be ffmpeg.
I did this with raw footage and a VESC speed controller dump to overlay a bunch of motor stats on a prior project, this was pre-AI and it was still only an afternoon project.
ffmpeg/image-magick do all the real heavy lifting out of the box.
Video's a bit more complex no doubt, but like you say all the pieces are there, SMoP.
https://www.dcrainmaker.com/2025/10/oakley-meta-vanguard-rev...
Yes, I've done the Garmin VIRB Edit thing, which is the very approach recommended by Jeff (NorCalCycling) in his tutorial videos on the subject. It feels like something out of 2005. It is incredibly labour-intensive and imprecise unless you're fortunate enough to be in relatively short criteriums where you've got the battery runtime to just record the whole race. Most real-world events and rides require one to turn the camera on and off at certain moments, which then requires _hours_ of stitching together clips and correlating them to GPS fixes from the head unit (in the FIT file), and quite imprecisely at that.
There has to be a more 2026 solution to this. All you need to do is correlate the footage to the FIT data points by timestamp, in the temporal domain.
If Garmin came out with one, it would absolutely annihilate this space. To the best of my knowledge, there is no competition that offers anything turn-key, though perhaps the best of my knowledge has not aged well and by now there is something. It's maddening.