ffmpeg provides the straight and easy way to burn-in SRT subtitle. By forece_style, you can customize your subtitle format like font, fontsize,outline,outlinecolor,borderstyle and so on.
ffmpeg -i test_30sec.mov -vf "subtitles=subtitle_en.srt:force_style='fontsize=25,Outline=2,OutlineColor=&H80000000'" out.mp4
Its result will be like below:
If you want to put dark block behind of the subtitle, you can do it easily by adding BorderStyle=4 like below:
ffmpeg -i test_30sec.mov -vf "subtitles=subtitle_en.srt:force_style='fontsize=25,BorderStyle=4,Outline=8,OutlineColor=&H80000000'" out.mp4