A year ago today I had 0 deathless champs, and now after a lot of grinding and some spending, I finally got Thanos.
I probably could've gotten him sooner, but it was only after the announcement of ascension that I realized 8s weren't coming and that the 7* Deathless Thanos was here to stay for a while. Had I chased him earlier, I would've done things differently, like choose the deathless piece from WoF instead of the titan nexus, or not sell one of the deathless pieces for random mats during a different event. Oh well...My advice for those starting their journey to get him is to do your research, map out all the available pieces in content carefully so that you don't waste any resources.
I don't really know how to play him on attack, but I've seen enough to know that he will be my first R5. I will have enough T5A after this week's coliseum, just will be short 2 T4As. Maybe I can grind another 1500 units by week's end to get one of those romance bundles, or maybe I'll just wait a little bit more, but it's definitely on the horizon.
Last piece:

Ready for the mausoleum:

Got him:

Ready for R5 soon: