If I read this right, setting off for the Golden Circle at 2, trying to get back to Sky Lagoon at 7
doesn't leave you much time. I did the whole thing in a day, so if you gave up snorkeling and
started off in the morning, you could probably see everything in a day without too much stress
and get to the Lagoon by 7.
Taking 38 -> 427 -> 43 to the Lagoon would avoid having to approach Reykjavik and might save
you some time. But there are always things that get in the way. I hit a road under construction
that wasn't designated on the map, and you want to leave time to veer off the main road and see
things you didn't know were there, like a small church.
You also don't say what time of the year you're going. Sunset will impact driving time a bit, as
well as crowds. Thingvellir may not be that crowded, but it is large and takes a while to get
through/around. Gullfoss is easily an hour or two. Maybe an hour for Geysir. If high season,
add time for crowds and traffic.