I don't think volume driven was necessarily bad approach when preparing for Europe, I think we just did it spectacularly badly under Koeman and Walsh.
Honestly, you just need capable back up players in a few key positions and a couple of utility men. A new pair of fullbacks, at least one new winger and a holding mid would probably do it for us next summer.
Its right in theory but in practice you risk doing a Nottingham Forest if you panic for volume.
Ultimately its good recruitment vs bad recruitment. Europe can help attract but sometimes/often no player is better than a bad player.