The shock towers are too narrow to allow any real caster tweaking.
Are you running camber plates? Were your camber readings equal both sides?
I'd start with a digital inclinometer and compare the L-R tower frame supports to see if one is bent. If they're good, the problem is probably at the bottom and could be a bent lower control arm which are pretty cheap and easy to change.
Next, also measure the L-R shock base tubes and compare sides as well. If they match or are really close to matching, the alignment shop is no good.
Next, is checking of the wheel in the opening. Straight edge the tires front to back to neutral with a string and measure aft edge of front wheel lip to front edge of aft wheel. Compare to side.
If you do go to the adjustable castor rodend setup, be aware it will shift the wheel forward in the wheel well opening when you add caster.
I'd replace the lower control arms 1st. But that's just me.
https://www.ecstuning.com/BMW-E36-32...n/Control_Arm/