I haven’t used /e/, so I can’t answer any questions about it, but the Brave browser is based on Chromium, which is the open source core of Chrome, but Brave has a better default configuration for privacy. Any security hole reported for Chrome will likely also apply to Brave.
The thing to keep in mind is that there are only three web browser engines + JavaScript engines that are up-to-date and maintaining good security: Blink + V8 in Chrome/Chromium, WebKit in Safari, and Gecko + IonMonkey in FireFox. Most of the the other web browsers in the market are based on Blink + V8, including MS Edge, Opera, Brave, Vivaldi, Comodo Dragon, Epic Privacy Browser, Torch, Yandex, etc.
The modern web browser has to be incredibly complicated to support 30 years of web standards and trying to keep up with the new standards and plugging all the security holes requires a huge team of programmers, which is why there are only three organizations that attempt to do it today. Opera and Microsoft found that it was too expensive to keep developing their own web browser engine which is why they adopted Blink + V8 from Google and why development of most of the other browser engines have stalled or been discontinued (KHTML + KJS in Konqueror, Amaya, Dillo, libwww, etc). It takes over 20 million lines of code to implement a modern web browser and creating one with good performance and good security is extremely hard. Mozilla found it so hard to create secure code with good performance and multithreading with C++, that they decided to create the Rust programming language. In my opinion, Mozilla now has the most efficient engine, but it struggles to keep up with the Google and Apple because it simply doesn’t have the resources.