Finding the overall transfer function and plotting root locus of a digital control system
By picking the a compensator I was able to get the result you want, try some (stable) poles cancellation or place the zeros very close to the poles. By picking different compensator you should be able to change how the root locus behaves. So, instead of using Optimization Based Tuning try a combination of:


Create a compensator D(z) with both poles and zeros inside the unit disc
Use rlocus(D*overall_ol_discrete)
Are the poles moving inside the unit disc? If not, go back to 1 and try big changes in D(z) 
Are the poles moving near/across the desired loci? If not, go back to 1 and make small changes to D(z)
Find for which gain \$k\$ the dominating poles are in the desired loci.


These are the results I got by using a
$$ D(z) =\frac{z-0.98}{z+0.9}$$