Having made several dozen end grained CB's over the last couple of years, I cannot strongly enough emphasize how tedious it's going to be to try to flatten a board with a RO 150, especially once the end grain has been turned up.  Maple, purple heart, walnut, my main choice of materials, are extremely hard and it takes a long while to flatten this way.  it's also very hard to be precise as well.  My suggestion is to get a decent planer and glue sacrificial boards to each end and then take very small bites.  The sacrificial boards will keep the end grain from splintering and I have had a lot of success doing this method.  Once they are flattened, then use the RO150 from 80 through 220.  They will come out as smooth as glass.

The router sled should work fine.  All you are really doing is getting it ready for the sanding.  You just don't want to end up with a board that "rocks" on a flat surface.  I always test mine out by laying it on top of a piece of granite.  Any inconsistences will be noticeable immediately.  Making end grain boards is really fun.  You should check out Andrei's YouTube channel.  MTMwood  He is a Russian who does breathtaking work and you can download plans very inexpensively.