I know it's a bit late but I just had the same issue myself and found a solution.
https://chrome.google.com/webstore/detail/bookmark-favicon-changer/acmfnomgphggonodopogfbmkneepfgnh/related
This extension lets you upload your own image while on a site you have bookmarked in order so you can change its favicon to your liking.
(I'm on Opera GX btw Idk if that makes any difference)