I used Subler to convert h264 video from mkv to mp4 so it can Direct Play via browser. It does its job beside the subtitles. As you can see, they are huge.
Subler how to - MKV to MP4 | MacRumors Forums
DVD/Blu-Ray conversion with text soft subtitles on macOS (2021 Update) – Eclecticism
Convert MKV to M4V for Free with Subler | OSXDaily
Subler: MKV-bestanden converteren naar M4V - appletips