Regarding the likelihood of Goblin packs in a Rift, the odds are that they will be after the Rift Guardian.
Here is why I state this:
Density has improved so much that the RG trends to spawn somewhere around the end of the 2nd floor. With about 6 or 7 floors per Rift if the chance of Goblin Packs are the same for each floor, then the RG spawn event isn't the midpoint to determine how likely a different Random event will occur. Clearly the Goblin Pack will more likely occur on the floors that are greater in number. This in my estimation is the 4 to 5 floors after the RG is killed.
I can't back up this will any real data or numerical formulae. This is purely an empirical assessment. But I do call BS to anyone that states that you aren't missing Packs if you don't do full clears. About 2/3 of the floors are left on the table if the Rift ends immediately after the Boss kill.
I do think that shards per time spent in a rift is a valid strategy. However, the Blood Thief spawn clouds whether this is optimal.
I don't offer this as a reason for any one to change their Rift exit strategy. My intent is to give the OP a logical a
reason for people to continue until the exit obelisk spawns.