Shanks isn't afraid of Blackbeard as a Fighter or his DFs or his Crew
He encountered Blackbeard's True Form & He knows it's Inevitable
He didn't ask WG to Defeat BB, He didn't ask Mihawk to help him, He didn't want WB to go after him
And as you just said, He didn't try to do it himself
Shanks is only Preparing for The Inevitable
For when Blackbeard's True Form emerges
I mean ask yourself, when did Shanks got Injured against Blackbeard?
The guy was on WB's Ship since ever, and No One knew that He was the one to Injure Shanks
How did Blackbeard secretly encounter someone as Famous as Shanks without anyone reporting it & without WB Pirates neither noticing BB's Absence nor Shanks Presence nearby, and also why did Shanks took Years to finally go warn WB about BB? Even though He always knew BB is on WB's Ship (Since They met as Kids)
Only Explanation is that Shanks encountered BB's True Form just like Gorosei or Imu, but this Form can only exist in Pitch Black, so BB Summoned himself to a Very Dark Place that Shanks found & Overwhelmed him, but Shanks didn't know who it is, it's like imagine if you encountered & fought Mars in his Full Zoan Form briefly, how would you know that it's Mars?
So Shanks spent Years trying to discover who that Person was, until He discovered recently it's Blackbeard
And He can't do much about it, except try to prevent that Monster from getting out
