It appears to me, that the best way of re-introductring the mangonel is by increasing it’s skillfloor of use; to make it fire on a target location only. This way, massed mangonels won’t be possible to micro effectively, at least not until a very high level, making them a support unit period.
By coupling skillfloor with the mangonel, the viable uses will more likely occur at levels where non-mangonel players have the capacity for effective strategical counterplay. Average level players won’t suffer from not having the APM to multitask and outposition a mangonel army.
Meanwhile, higher level players will be able to enjoy high-risk, high-reward micro investments, where the mangonel will be a more two edged sword than before, and add more spice to pro-games.
I believe this is the way to go, because anytime a strong unit is balanced out with a slow movement speed only, the unit is going to be over-represented and OP in lower leagues; since the counterplay of slow units - working around the enemy’s army rather than engaging it, is something that’s hard to do and limited to very high levels of play.