The new, larger diameter disc is acting like the rudder on a boat.
I assume your disc harrow is a secondary tillage tandem disc harrow, not a primary tillage, heavy offset disc harrow.
Yes the disc harrow (leinbach) is a secondary tillage. 16 discs total. 4 discs per row.
1) Adjust Three-Point-Hitch Top Link so less weight is on set of discs containing new, larger disc, more on the second set of discs with all old/uniform diameters. Extending Top Link will put more weight on the rear set. Shortening Top Link will put more weight on the front set.
The new disc is on the outside of the left front row. So I should extend the top link to place more weight on the rear set?
2) Adjust disc gangs to less aggressive settings; i.e.: gangs more parallel to one another, less "V" configuration. This can be a struggle if disc harrow frame and bolts/nuts are rusted. Perhaps adjusting just the gang containing new, larger diameter disc one increment will correct tracking. Gangs need to be in approximately symmetrical settings so dirt is moved IN and OUT equally.
The gangs are currently adjusted just short of parallel. I am not following how to adjust just the front left gang since the adjustment bar is bolted to all 4 gangs.
You do not specify your tractor. Most stabilizers are adjustable and, yes, that will help, but try adjusting Top Link and gang angles first. A properly adjusted tandem disc harrow should follow tractor with stabilizers adjusted "neutral".
Check axle nuts to be sure they are TIGHT so the discs on one or both gangs are not wobbling around when pulled.
Add your location and tractor, please.