How do you edit subtitles that are embedded in a video?

[SeaofBloodRedRoses]

Okay, so this isn't necessarily VLC specific, but there might be a built-in way to do it or something, and I'm not sure where else to ask.

Basically, I have a bunch of MP4 files with built-in subtitles.
Now, I like subtitles, but I don't always watch with them on. The problem is, when I leave them off, there are no hard subs for the foreign language parts.
I have another srt file for the first batch of these videos that I found online, but they have flaws (timing, spelling), and while fixing those isn't particularly an issue, it is annoying. Plus, these are only for the first 10 out of about 60 videos. The other 50 I can't find online. I can find full subs, but that's just more work for me to delete everything that isn't foreign languages (meaning I need to watch all of those videos and pause and edit whenever I come across a line, deleting everything else), as well as double check spelling, punctuation, and times.
Another problem is that that srt file isn't built-in, so I need to open it as well whenever I play a video, which is rather annoying.

My first question then, is how do I edit built-in MP4 subs? I don't want to actually edit them, since I want to keep a file of full subs incase I want to watch with subs. I do, however, want to copy/paste those subs into a separate file and just delete everything that isn't in foreign languages. Having official subs would be a lot easier to work with. All I would need to do is delete. Of course, I would still want to keep them built-in to the video, so I could use them. If this isn't possible, then I guess I could take the massive effort to edit them all manually, but then my second question still needs to be answered...

My second question is the more critical one, and the one that directly relates to VLC. How do I take a subbed file and either set it to automatically open whenever the video does? Or at the very least, set it as one of the track options instead of having to go into explorer to fetch it every single time.
I don't mind not having it embedded in the video, as long as I don't need to go looking for it each and every time.

Bonus question! The built-in subs are tiny. Not unreadable, but they might be difficult to read for someone else who is sitting far away (I have good eyes, so I'm good, but I don't always watch alone). The ones I found online, though, are big. Tiny bit too big. Not something I'd worry too much about, since they don't come up that often, but still... a little big.
How do I edit subtitle size? Are the sizes for these both editable, or can I only edit built-in ones? How do I edit each one of these?

Thank you!

[mederi]

You need to demux the subtitle track out of mp4 container, then edit the subtitle file in a subtitle editor and then remux video, audio and subtitle tracks into mp4. ffmpeg, Mp4Box with GUI (my MP4box GUI, Yamb)? What is the format of subtitles in your mp4 (VLC menu: Tools > Codec Information)?

[SeaofBloodRedRoses]

You need to demux the subtitle track out of mp4 container, then edit the subtitle file in a subtitle editor and then remux video, audio and subtitle tracks into mp4. ffmpeg, Mp4Box with GUI (my MP4box GUI, Yamb)? What is the format of subtitles in your mp4 (VLC menu: Tools > Codec Information)?

It says tx3g.
I'm more concerned with getting subs to autoplay with certain videos though. How do I do that, instead of needing to manually select it each time?

[mederi]

Just name a subtitle file according to a corresponding video file and copy it into same directory: "video file.mp4" and "video file.srt".

[InTheWings]

Get MP4 authoring too that can switch forced subtitles flag and delete tracks.
MP4Box/ gpac