I need to track cardio with terrain data, but I'd rather not trust Strava with my privacy. I know there are some alternatives, but which one is the most reliable and feature-rich? I don't have smart accessories, just and android phone. Preferably, I'd like an app that let's me track added weights for calorie purposes (like rucking).
Garmin is like the Apple of the fitness world. Very much not open source, but broadly privacy-respecting.
And like Apple, the reason they do this is because they don't make their money from data harvesting, they make it by selling high quality hardware at a premium. And depending on your perspective, either unlike Apple or to a much greater extent than Apple, Garmin pushes their hardware by artificially restricting their software. You can expect to get maybe one year of feature updates on your thousand dollar bike computer or running watch, and a few security updates after that. Some of those limitation might be because of genuine hardware limitations (e.g. my Forerunner 935 not getting Garmin Pay because it lacks an NFC chip), but many are purely because they want you to have as much incentive to upgrade as possible.
This is it: https://github.com/Flomp/wanderer
So far it's just a way to host your own trail maps, but hopefully it will expand to have more functionality like tracking your walks/runs.
It does not mention FOSS anywhere, so it is highly unlikely it is. But I have to say, their privacy policy is not bad (not great either). Much better than most nowadays.
I like / use fitotrack (on the rare days i go out for runs). It has altitude / height of my runs, as well as custom exercise setups, and local exercise data backups. I'm quite happy with it as a tool for tracking cardio.
Runner Up, on f-droid is great for measuring speed and the like. I assume you meant running and gps. Other sports (e.g. gym, weights or functional training) got other apps. I'm not logging anything, just use TimeR Machine for everything. As for the food, I have no experience but there are a couple of apps on F-droid like FitBook or SECUSO's food-tracker. Good luck!
Maybe you find it. But do keep in mind that FOSS projects tend to be smaller than commercial privacy nightmares. So, if you could live with the functionality being split across apps, it will be more feasible. It's also easier to maintain smaller apps, so there's many benefits overall..!
If you are serious about training, Training Peaks is probably a great alternative. Strava is the Facebook of activity apps so not really great if you're committed. There is a free plan for TP, but I'm not sure what the limitations are...
I pay for it and can track the history of the key metric that matter to me such Power, Heart Rate, Training Zones, Freshness, Power bands etc. There are a LOT of analytical graphs to provide insights into your progress. The key is the more data you collect, the more value there is. i.e. Sleep tracking, weight, HRV, cadence etc.