[mr-b]

Hmm ... "just". ;-)

I've no idea idea how to do any of that. All I want is for a simple setting to enable subtitles to automatically display if they are available.

[sauromayne]

https://github.com/a4k-openproject/a4kSubtitles

A4K subtitles addon does a good job of managing subtitles. I use it to auto enable embedded subs, and if I don't have embedded subs then I use it to search 6 subtitle providers at once to find subs to download.

[Karellen]

All I want is for a simple setting to enable subtitles to automatically display if they are available.

You won't get it.
There are many types of subtitles - Forced, General, SDH, Commentary, Info, how does Kodi know which one you want?

Subtitles need to be correctly tagged. When they are correctly tagged, then Kodi knows how to deal with them. Problems come from all the available video downloads that have poorly tagged subtitles.

As @DarkEther states, you need to tag your subtitles correctly.
Don't know how to do it? Install MKVToolNix. Once you select the required video files, they will be imported into MKVToolNix, then you deselect the streams you don't want and tag each one as shown in the images. In the images, I have chosen to keep all the streams...

View Spoiler

Once you do this, Kodi will become your best friend. Everything will just work as it's supposed to. Hope that helps.