Don't try to delete the packages that are built into the image you flashed. That will, counterintuitively, actually consume space and it will not free anything up. If you want to remove packages that you won't be using, build a custom image to save space in the actual image (and you can also include the things you would otherwise need to install).
I don't think this applies to the OP with ath79 21.02.1, but if you're running snapshot builds, the attended-sysupgrade will automatically squash up a new ROM with your desired package configuration, including removing standard built-in packages that you don't want. Add or remove packages, then request an upgrade and they will all be put in the ROM instead of the overlay.
Except that .opkg files are compressed archives, which will expand to a larger amount of files, then the jffs2 system compresses the data with a different method while storing it in flash-- so the amount of flash occupied by a package can't be readily predicted.
Note that one of mc dependencies is glib2, which is 1.1 MB compressed.
The safest way to do this is use Image Builder to build in what you want then check the size of the resulting image (this also re-compresses the packages into a squashfs using lzma, which is highly space-efficient). You need to leave at least 500 kB of flash free for the overlay.
tmpfs is a RAM disk. Everything stored there will be lost on a power cut or reboot.
The USB extroot is the only viable option with this router... I did it and it works without issue and you will have more space than you will ever need for packages... but it is tricky to setup a bit and make it functional still effortless if you are determined, you can uninstall some packages (I don't remember the list right now) to make it possible to do the extroot trick... I recommend you to use WinSCP or Putty to do this, if you still need help I will try to gather you the required details.