As far as I can tell the Table of Content entry text is always Georgia by default. In firmware 4.1.7729 (but not fw 3.19.5761 or earlier) it is possible to patch the firmware to specify a different font but it is a job that could best be described as "would suit enthusiast".
I know this is possible because I patched my AuraOne to display ToC entries in the built-in Avenir sans-serif font - not because of missing glyphs but because I find sans-serif easier to read in the ToC. I suspect a sideloaded font would also work.
Similarly, it is also possible to change the font used in various parts of the GUI Book Lists e.g. Book Title, Series, Author. Changing to a built-in font works "better" but you can also use a sideloaded font if you are prepared to take the extra step of opening a kepub after every full reboot to force the system to "see" your sideloaded fonts again. Many of these GUI items can also be patched in fw 3.19.5761 as I have successfully experimented with changing my Aura H2O and GloHD to display Title, Series, Author in a sideloaded font at sizes more to my taste.
There are currently no "magic button" solutions which make these GUI patches as easy to install as GeoffR's patches to other parts of the firmware.
If you want to investigate further you would first need to read and understand
pipcat's alternative method for patching the nickel file. tshering's method for applying Modifications for Bad Eyes is also a possibility. If you can get that working, I am more than happy to pass on what I've learned about which bits need to be changed to affect the look of the Book Lists and ToC.