The official version of Brave available on the AppStore should be on WebKit, but with the current agenda of European lawmakers, we can expect permission for sideloading apps on Apple devices fairly soon.
On top of that, not only is sideloading already available to all users via apps like AltStore, via the ooky ways, but a lot of tech-savy people are taking advantage of it and Brave users may well benefit from it.
I’d like to suggest you make builds with Chromium engine version on iOS/iPadOS and distribute them via the official website.
As a downside to the current state of operations I see the need to re-sign apps every 7 days or they’ll be gone along with all the data, but that’s a risk I’d take if it meant a third-party browser engine.