Nice report, cristian! Since you lowered the ride height and it found it helped with the "abruptness", it sounds like you were topping those shocks out as I suspected (not "bottoming out" as you report, as if you were bottoming out the shocks, lowering the ride height would make the problem worse.)
As for whether 1-2mm of "loose" spring travel when shock is at full extension would be a problem, really assuming you're not taking your car off jumps and getting it airborne, the most common/likely scenario where this could be a problem is when jacking up the front end and unloading both front tires at the same time. (Even in an extreme cornering situation with high-grip tires and in a competition situation, where you got one side of the car completely off the ground and the car into two wheels, the anti-sway bar would provide some compression to the unloaded side, so wouod unlikely allow the springs to become loose on the perches.) High-pressure monotube shocks have more nose pressure than low-pressure twin-tube. (Are these monotube or twin-tube shocks and can you find a nose pressure specification? This is the pressure the shock requires for compression to begin.)
If the upper spring perches have a tall enough spring-guiding sleeve such that the springs always reliably guides onto the perches, (and it's hard for me to imagine this being less than 1-2mm, usually they are more in the area of 1/4"-1/2") I'd say you probably would have no problem even if the springs do occasionally become loose.
Usually adjustable spring perches I've worked with have a locking feature of some sort, like a set screw or jam-nut - if yours do not, I'd suggest marking their orientation (like using a marker or paint), and monitoring them periodically (e.g. when you change tires) to make sure they stay put.
If loose springs were a problem, helper/tender springs could be a fix, or altering the strut tube or strut mount could be a fix. Or even zip-tieing the spring secured to the upper mount, although it's possible the spring could cut the zip ties if they compress enough.
You could also try raising the ride height slightly, (by, raising it back up by 1-2mm
) such that the springs become unloaded right as the shocks reach their travel limit. It doesn't seem like you'll need helper springs. They take up probably 1" or so. If you're looking to maximize performance, they are best to mount above the main spring (rather than below) so they're not unspring weight.
Nice work and good luck dialing in the rear also.