MU has an alternative for onana. Onana was a priority, but it doesn't mean MU shall only focus on him.
I prefer to see MU get one from these keepers, Justin Bijlow or Kevin Trapp.
Kevin Trapp played awfully, Bundesliga clubs easily to humiliated him.
Justin Bijlow played good in Eredivisie, but his recent performance in Euro was not good. But I think your speculation could be right because he's a Netherlands player, it's a privilege as Ten Hag is favor to sign Netherlands player.
If there's no high quality player, then the another option is looking for Netherlands player.