I'm getting the "srt" files from "subscene.com". The hardest part of this method is figuring out what is the best (correct) srt file for the movie. It appears that people upload their SRT file to the site, so there are like 50 files for the same movie for the English language. Some are for the DVD movie some are for the BD version, some are labeled as Theatrical Cut version, etc. I just picked two that looked correct and so far it seems to be fine.
For many movies you won't find it, or it will be wrong. Even if you find something, you have to validate it's correct. Yes, dropping a SRT file in a folder is easy. Finding the right one, if you find it at all, is hard and time-consuming.
I suggest you try my method. It is simple, accurate, and takes less than 5 minutes to do, which is less time than you will spend looking for a new correct track on the web.
The app will automatically identify which subtitle lines are forced. The only info you need from MKVtoolnixGUI is the track ID for the main subtitle track, that contains all your forced and non-forced subtitles. And in the case of a DVD rip, likely there is only 1 subtitle track. If you don't like using the command-line mkvextract and identifying the TrackID in MKVtoolnixGUI, then you can download MKVcleaver, which will do the extract for you graphically with 2 clicks.
What you're doing is harder, slower, and less accurate than what I'm giving you. But suit yourself.