It doesn't support HDR for example and a lot of websites are "optimized" for chrome or refuse to work unless you change your useragent to chrome

Because, for years, Chrome and its forks were basically the only good browsers out there. Both Internet Explorer and Firefox were in bad shape at the time.

Internet Explorer never recovered its reputation (and Edge itself is a fork of Chromium, anyway), and, by the time Firefox recovered, too many people were integrated into the Chrome ecosystem, and too many of those people believe it'd be too difficult to switch (and most don't really care since Chrome's issues have yet to personally affect them).

I am using Brave on iOS mainly because of its superb YouTube support - It has a built in ad block, can download videos offline and play minimized. Is there any way I can achieve this with any other browser? I would switch immediately.

Because Firefox on mobile is a pile of shit to be honest. It works great on desktop, but the mobile experience is subpar. For a chromium browser, Bromite used to be an option, but it seems abandoned now. Brave is one of the few Foss chromium browsers left that also supports adblocking.