I don’t pretend to know what Mueller is thinking and what he has. However, it certainly appears from the widening of the scope of the investigation that the complete exoneration to which Trump thinks he is entitled is an unlikely outcome. At best, Mueller may find that there is plenty of smoke, but not enough proof of a fire; at worst, he may determine that Trump committed an indictable offense.
In my view, it is more likely than not that Trump will take the necessary steps to fire him in the fairly near future. Sure, there will be an uproar, and comparisons to the Saturday Night Massacre. That said, he knows that the base supports him and considers the investigation a witch hunt, and that the GOP Congress is far too afraid of the base to impeach him. Furthermore, the public can easily be distracted by North Korea, or Iran, or the latest outrage in the culture wars. From his perspective, therefore, the benefits of putting an end to the investigation probably outweigh the risks.