1. As a consumer, I would like to have such an editor / viewer for my phone, is there something alike for traindown?
2. Functionality-wise, wouldn't a nice editor/viewer app necessarily be a subset of spreadsheet-like app and isn't traindown then competing against such a more general app? (Not that I know a single FOSS spreadsheet app, so I'm speaking hypothetically)
I think the issue here is that you never really stuck to a single app and used it long term. I was in the same boat, I pretty much tried all the apps, bought an Apple Watch, but nothing really satisfied my needs. In the end, I just stuck to one app (Strong), and kept using it. It's not perfect, and there are things that I'm missing, but in the end, it always me to track all my lifts and shows me the progression. It's indeed something that I could quite easily do with a notebook (although I won't have the fancy graphs), but I already have my phone with me to listen to music, so might as well use it to track.
Along with rough tracking of calories -- I ignore things like green veggies and coffee -- and weight and I'm golden.
As a powerlifter, I completely agree with the OP. When I first got into resistance training, as a techie, my inclination was to try all the apps and gadgets. 3 years later and I realised that the old timers had it right, the best tool for resistance training is a pencil and a notebook, maybe backed up by a spreadsheet to track long-term progress.