Slow cooked steak

No, the fat will not render at about 50 C (122 F).

However, you said core temperature, which implies the surface temperature will be higher assuming you are not cooking in a 50 C oven (which you should not, for safety reasons). If you are pre-cooking the steak at, for example 120 C (250 F), the surface will be hotter by the time the center reaches your target temperature, so you may get some rendering.

Still, there is a quite simple answer: trim the excess fat. What remains should char and develop a good flavor when you sear it on the grill.

...or, you could sear the surface on a very hot pan. Place in a zip lock bag. Remove air by submerging in a pot of water and sealing (before water enters the bag) and drop in a water bath. 50C will be virtually raw, though. You might want 54-56C. If you don't have an immersion circulator, Google "ghetto sous vide", you might be able to rig something up in your kitchen. Thickness will determine length of cook time (there are a few apps and lots of web resources for this), but generally the longer a rib eye cooks, the more tender. At some point, after several hours, it might get unpleasant and mushy. After the cook time, sear a second time on very hot pan or grill, season, enjoy.

I regularly cook beef the way you describe. I commonly make thick ribeye steaks, about 2-3cm, and put them, salted, directly on an aluminium baking sheet in the oven set to 55?C. Then I leave them for about 30-60 minutes until it is time for dinner. At 55?C they are piping hot, red and bloody, and juicy and delicious with the connective issue all gelatinous. The fat becomes soft and translucent but is not rendered. I sear them 30 seconds per side in a heavy and hot pan, before serving. My wife occasionally wants it more well done and will put her piece back in the warm pan.

Now we will get to the problem with the "big piece" - I assume you mean 2-5kg, a 2-4 bone rib roast. At a 55-60?C oven the inside is not going to heat to 50 until the outside surface has been warm for dangerously too long. All the exterior surfaces of this roast will be coated in bacteria (like everything is) and there is a limited time you can hold these warm temperatures.

As a result, when I do a big thick piece, I start with a hotter oven (120-150C?) and turn the temperature down to about 65-70 when the meat goes in. Allow the outer surface to brown, sterilizing the exterior. I then monitor with an internal probe until the roast is done (for me, 60-63?C).

So, the barbecue step in this case serves no function anymore. It probably won't be exposed to the smoke for long enough to impart any flavor.

Note: when you cook beef this way it does not need to "rest". If you let it sit out it will simply get cold.