If flat and softer is the main objective any of the tipped 200gr 10mm/40cal bullets will be hard to beat. I use mostly Hornady 200gr SSTs but i dont think they offer that bullet with a 50cal sabot. TC does though under the Shockwave series.
Although im not a huge fan of PRs bullets (such as Dead Centers) some of them are also very flat shooters and they offer several in 50x40s from 195gr-250gr+.
Barnes offers a 195gr flat based Spitzer MZ but again you will need to buy the sabots for the 50cal instead of the 45.