I see no alternative to current icons because ASCII art
characters have own problems explained by Eli in
https://lists.gnu.org/archive/html/emacs-devel/2019-09/msg00268.html
This is why the tab-bar moved from Unicode characters to images,
but another attempt to use SVG failed as was demonstrated in
https://lists.gnu.org/archive/html/emacs-devel/2019-09/msg00414.html
But in any case feel free to try other options.
BTW, if you want to see a funny effect, try typing 'r' when point
is on the arrow in the customize buffer - it will rotate the image :-)
There are other image keys used in the customize buffer,
e.g. '+' will zoom the image (function 'image-increase-size'),
so maybe you could just increase image size instead of using
2x image files.