MVC Repository Insert Using async
The best thing you could do is let EF do what it's good at... You don't need to save every single time you add an entity - let EF track the changes in its DbSets repositories and then save the changes in the 
DbContext unit of work.

As a more general point - why aren't you saving the whole DomainModel at once? I'd expect to just be able to do:

context.DomainModels.Add(someDomainModel);
await context.SaveChanges();


I've hinted at it but I'll say it outright - you get very little to no benefit in writing Repositorys wrapping the db context. It's just not worth it.

I'm not going to comment on the bad naming because I'm guessing that this is actually just an example of your working code.