Hi CDG,
Tulamben is about 2.5-3hrs from 'downtown' Bali-Airport area. Staying in Tulamben for 4 nights will get you diving the Tulamben sites (liberty wreck, drop off, coral garden), Amed, and Seraya (muck) as they are only about 1hr away from Tulamben. Staying in Tulamben is a good plan because you will get the early morning dives in peace, just before the dive groups from Kuta/Sanur starts to arrive. I dived the wreck twice and going to come back on April and going to try Seraya as well.
Mimpi is a good resort, I have no clue about the dive op tho. Last time I went to Tulamben, I used Bali Stingray, they are based in Sanur. This time we going to try a newish resort "Ocean View Dive Resort Tulamben", about 50m up from the wreck entry point. also have no clue about the dive op, they say they are working with Deep Blue Tulamben.
Ubud is def far from the airport, about 3hrs If I remember correctly, def not good if you have a morning flight to catch.
Just to add to your confusion

, maybe you would like to dive the purposely built underwater temple? It is in pemuteran. If you do go there and go to Tulamben, you would be driving around Bali Island. Pemuteran is 4hrs drive up from the airport, and from there to Tulamben it is about 3-4hrs to the right, then from Tulamben back to the city/kuta. If you see Bali's map hopefully you understand. We did that safari once (Secret Bay-Pemuteran to the Wreck-Tulamben).