It´s posible, for future updates, change all letter avatars to an html element with css for improve load performance with minor request? Sorry for my english, i´m not native speaker.
This is a nice example:
https://codediode.io/lessons/198632-default-avatars-with-username-initials
This was discussed a lot in the past. There is no way to get a clean and consistent letter avatar using HTML and CSS, browsers have quirks and it will not be consistent across all browsers.
Devil’s advocate here: why should they be pixel-perfect consistent across browsers?
Because even as images, when they weren’t, there were floods of topics about it not being pixel perfect…
For my the big problem is the browsers 
Im sorry, i dont search this topic first if you discussed in the past, only small search.
SVG is a possibility but email clients tend to barf hard on SVG. There is a reason we made it the way we did and a ton of research went into it. Feel free to search.