I've been using Piper for this. The quality is (in my subjective opinion) as good as the TTS built into MacOS is, it's open source, and it's so fast that you can run it in real time on a raspberry pi. On a real computer I can generate a whole audiobook in about 20 minutes.

What I do is I split the book up into sentences, generate speech for each sentence and at the same time turn that sentence into subtitles. Then I combine the two and stitch them all together into a mp4 container with audio and a subtitle track using ffmpeg. mpv (and think VLC) can display subtitles synced to audio playback even when there is no video track.