Hi, I find the audio capabilities of Shotcut more than adequate for music soundtracks on videos and like yourself I do my own soundtrack for my videos. Other peoples workflow may differ but mine is basically as follows.
- Do a edit of the footage, place in timeline, complete the visual aspects of the video. One important thing I find is to leave approximately 2 seconds of each video/image clip beyond the in and out points so that can can increase/decrease a clips length if you need to for timing etc.
- Sit down in front of my DAW and run the video, whilst composing the soundtrack. If you want precise timing use the frames counter as a stopwatch etc. Record the soundtrack using the DAW, I generally find it easier to try and the soundtrack at a constant volume and use Shotcut to alter the volume later. Once happy export it as a WAV file (lossless so less decoding required).
- Add the soundtrack WAV file as an audio track. If the timing is not precise you have leeway to lengthen/shorten clips without worrying about running out of clip/image.
- Using the Gain filter and keyframes on your soundtrack to increase/decrease volume of audio as required.
The best analogy I can think of is that the Gain keyframes is a remote control of the audio fader, you can control when it goes up/down. Same for crossfading across 2 tracks, place the Gain filter on each track then use keyframes to crossfade the audio.