I just went through a very similar use-case working with TS files. MoritzLost has the key point but as ffmpeg options can be quite difficult to sort out when you are first getting started, here is a more detailed answer.
For a single input mp4 file and single input SRT file, we can keep all streams from both files and merge them into a new mp4 ...
To use newlines in the built-in Subtitle filter, you must specify the lsp (linespacing) argument:
subtitle("First line\nNext line\nLast line", lsp=12)
See the Subtitle wiki page for more info on the argument.
This is a workaround which converts the files to another container format. It happens almost as fast as your hard disk can write data because nothing is re-encoded.
use MKVToolNix (specifically mkvmerge) to combine the mp4 files and their srt files into mkv files (perhaps you can automate this with a loop). I never use this tool via command line and cannot ...