Hi Harry,
Glad to hear ACF is helping you out 🙂
I see what you mean here and my only concern is wether it is possible with the way oembed works in WordPress at the moment.
The oembed field is actually really simple. All it does (for the actual oembed part on frontend) is that it returns the value from wp_oembed_get() which is a WordPress core function.
My concern is that rather than it being ACF not recognising an url with ?rel=0 it’s the WP Core function. And since it doesn’t take any other arguments than width and height (apart from the url to embed) custom controls for this in ACF couldn’t do anything.
