I just tested the cast bar for Sleep. Looks like the sleep duration is still the same if you release it early, but the “Attack Threshold” is lower if you don’t finish the casting. I guess my Quickcast is also useful for casting sleep. 
300 on the guage at Rank 6 Necro 1, not sure if it increases at Necro 2. You get the most bang from Flesh Hoop and Cannon if you use Linker’s Joint Penalty. Hoop is more Flesh-thrifty.
If you know how to “pull” the summon and you have Templeshooter, then you don’t really need C2 unless you really want skills on the summons.
I’m adding 2 more options on my list of builds:
Wiz3-Link1-Thauma-Necro-Warlock
Wiz3-Link1-Thauma-Necro2
I just thought the buff I’m getting from Linker1-Taumaturge1 is more worth it than the additional links I get from Linker2