Getting the soul stone is highly personal
I mean, there's a reason it's called the soul stone. Further, the sacrifice must be made to get the stone - it says nothing about payment-free transfer. Maybe a minion couldn't even get the stone. In particular, note
Red Skull: A lifetime ago, I too sought the stones. I even held one in my hand. But it cast me out, banished me here, guiding others to a treasure I cannot possess.
Those who do not truly desire the stones for themselves might not be able to get the soul stone.
Red Skull: The price. Soul holds a special place among the Infinity Stones. You might say it is a certain wisdom.
This could be something like the security check you suggest in your question.
A comment on this answer also makes a good point - Thanos seems to be unable to just take this stone by force. He has to make the sacrifice that he does to get it. This makes it pretty clear that the stone has some sort of wisdom, as Skull says.
Rather awful minions
Let's go over some of the minion record up to this point in Infinity War, shall we?
The people closest to him - his daughters - have betrayed him. He ends up having to torture one to get the information as to the location out of the other.
Minion #1, tasked with getting the stone from Dr. Strange at the very beginning, died. Whoops. (Being a minion for an angry titan has a high turnover rate...)
In other movies, Loki failed and Ronan failed to get the stones
I could keep going, but hopefully it's clear.
In summary, to quote what might possibly be one of my favorite comments ever:
He's not exactly batting a thousand on proper minion support at that point.
Thanos' plan is nearing completion, and, to quote Time:
He seems to be making much quicker progress by just tracking down the darn things himself. [...] Now also happens to be an opportune time to attack earth, what with the Avengers team broken up.
Don't want that window to close. Once you really start, you have to move quickly to truly make use of the element of surprise/off-kilter-ness.
Think very carefully about what Skull says to Thanos:
Thanos: Where is the Soul Stone?
Red Skull: You should know, it extracts a terrible price.
Thanos: I am prepared.
Red Skull: We all think that at first. We are all wrong.
Thanos thinks he's got it. The sacrifice he ends up having to make pains even him, the guy who's set on killing half the universe - he's not expecting it at all.
All bolding here my own.