Reykjavik to Diamond Beach and back in a day?

If that's what you want to do, then do it. If it were me, I wouldn't do it because there is a whole lot of wonderful stuff to see between Diamond Beach and Reykjavik.

But this is your third visit to Iceland. Besides the Golden Circle what else have you seen? Why are you spending all three nights in Reykjavik this trip?

I personally wouldn't do it because there is so much to do and see between Reykjavik and Diamond Beach. Highlights for us were kayaking in one of the smaller glacier lagoons and viewing the icebergs floating in Joksularlon Lagoon. There are so many places to stop off and hike to view the glaciers up close. To be honest, we thought Diamond Beach was a bit overhyped (we liked all the other places/ experiences better) but there also wasn't a lot of ice on the beach when we were there. BUT, as the previous poster said, where else have you gone in Iceland on previous trips?

Thanks for the information. Where we have been before is on the golden circle: Thingviller, Geysir, and Gulfoss falls, Blue Lagoon, and exploring Reykjavik. I would really like to see glaciers up close but thought that was further out than Diamond beach.

I would not do it. To give you an idea, we had been to Iceland 3 times before, and then last fall on our 4th trip, we headed back up the south coast. We had great weather, so no issues with that, but still we only went as far as Vik because we didn't want to have to bypass everything we saw just to get to our destination. Google maps times are really off for Iceland, IME, so I would add another hour to that at least, and that is without stops. So minimum 10, probably 12 hours round trip without stops. You will get tired, and that puts everyone else on the road with you at risk.

You a see a glacier much closer than Jokulsarlon. Just west of Vik is Sólheimajökull. There is a parking lot there and you can walk to the toe of the glacier. If you do a tour, then they outfit you with crampons, an ice axe and a climbing helmet and you actually go up on the glacier. So that, IMO/IME, would be a better use of your time for what you are asking. Still, the problem is that there is so much to see along the way. I would suggest not staying I Reykjavik the entire time, but rather stay up along the south coast for at least a night or two.

I've done a guided hike on the Sólheimajökull glacier. It was really cool. The guide talked a lot about the shrinking of the glacier in recent decades. The hike lasted maybe 2.5 hours. Unfortunately I don't remember the name of the outfit we used, but google tells me there are several that lead hikes there.
We did it in late May 2019, part of a loooong day of driving from Reykjavik to Vik and back, with stops along the way. The glacier was one stop; we stopped at the black sand beach and a couple of other places. There are a number of picturesque places to pull over along the way. There were some cliffs with a small parking area and WC. We parked there, hiked around a little bit, and saw some puffins. We ate dinner in Vik and got back to Reykjavik around midnight. Of course we had nearly 24 hours of daylight, so we were never driving in the dark. I considered Diamond Beach, like you, but that was our first trip to Iceland and it was just too far for us in a day, even with an intrepid 21 year old doing most of the driving. We were only there for four days, so our base was in Reykjavik.

We also did the guided glacier hike there. We went through Troll.is for our tour, which was last February. Our tour was to be 3 hours, but our guide asked if we wanted to go further and our group agreed, so we did. I think we tacked on about another 30-45 minutes total. But as mentioned in the post before mine, it could be shorter. It was a very long day, and we were not even doing the driving.

We are headed to Iceland staying in Rekjavik for 6 days in March. My daughter really, really wants to go to Diamond Beach and thought about renting a car and doing the drive all in one day. Finally after some research and realizing that it will still be dark for a good bit of that long day and it will still be winter with weather being unpredictable, we decided to do a guided excursion to Diamond Beach and let someone else do the driving. We booked through Reykjavik Excursions. Since we haven't yet been I cannot comment on whether it is worth it or not but wanted to let you know there was another option if that is where you want to go.

Google maps is NOT accurate for this route. Add at least 30% to what it says + a little more. Traffic (especially in July/August), wind, trucks, one-way bridges, etc., make this route very unpredictable with it comes to how long it will take. There is A LOT to see on the way there and you will miss so much of the spectacular things that are along that road. Drive out to Vik, spend the night, and see everything along the way. One way would be far too rushed and exhausting for me.

I wouldn't do it. When you're driving between Reykjavik and Diamond Beach, there are so many places to stop. It's all Glacier! Glacier! Glacier! right off the side of the road. Plus, you'll want a few hours to go kayaking, take a boat tour, etc once you're there. Can you just cut a few nights out from Reykjavik instead?