I just use `xsv` for this. `xsv concatenate` and `xsv join` do a pretty good job. Blazing fast: https://github.com/BurntSushi/xsv
Funny thing is several years ago I made a very similar tool to what you pointed out above written entirely in bash/awk and is also blazing fast.
That's where I got the idea for this online tool. I wanted to create the same exact concept of working with files locally without having to open up a terminal/console.