So you can do the time sliders without having them in a mosaic dataset on an image server?
yes, that is correct. In the animated gif above, I used 3 different raster layers. They are not inside a mosaic dataset. And it is a map service not an image service.
What if I made a tile package (.tpkx) of of a time enabled mosaic dataset?
I'm not an expert on tile packages. My understanding is same as what @George_Thompson suggested above. Tile packages are not time aware.
In you case, once 11.3 is available, all you need to do is add your raster layers in a map in Pro, make them time aware, and then share that out as a map service.
I hope this helps and please let us know if you run into any issue with this workflow while (I'm assuming) you will be testing this out with 11.3 enterprise.