One Day or Two? — Quick Answer
Can you visit Jökulsárlón from Reykjavík in one day? Yes — but it's a 13–14-hour day with around 10 hours of driving. A one-day guided tour suits visitors who want the headline sights (Seljalandsfoss, Skógafoss, Vík, the lagoon, Diamond Beach) without organising the logistics.
For Katla Ice Cave, Skaftafell/Svartifoss, Fjallsárlón and a proper glacier-lagoon boat tour, plan two days with an overnight in Vík or Kirkjubæjarklaustur. That's the version of this route that stops feeling like a long transfer and starts feeling like a journey.