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.
Manchester Unites do have other alternatives if they fail to get Onana. Even now, Manchester United's offer to get Onana has been rejected by Inter Milan. Because Manchester United made an offer below the price set by Inter Milan. Manchester United made an offer of 40 million euros and a bonus of 5 million euros and was rejected by Inter Milan who wanted 50 million euros plus a bonus of 10 million euros. And I think the Manchester United side did not just give up. They will surely make another offer. Because Erik Ten Heg really seems to see Onana as the most suitable candidate for the pattern of play that has been built by Ten Heg at Man United at the moment. But if the next bid fails, then maybe then Ten Heg will switch to other Alternative options.