I have the Tonal Energy app on my iPhone and use it for tuning. I can see using it for long notes with a bow - it takes a bit of time for it to settle into a note. I don't use it for that, like others here, I prefer to just rely on my hearing.
There are plenty of great exercises you can do to improve intonation that don't rely on electronics. For me, It really comes down to training my ear. If I can hear it, I can correct it. I spent a good part of the covid lockdowns working with the Ottman's sight singing book. I'm a horrible singer but regardless, it helped me get pitches correct and really helped develop my hearing.
On the bass, one of my current favorite exercises is to play a scale starting anywhere around the first position E string (like, F or G maj) and going up two octaves - hold the final note and then play an open note and compare against the open note. When I first started this a few years ago, I found my self drifting over the two octaves. Now, I can do it and fairly consistently end up within a hair of the correct pitch.