I definitely think the changes make sense and are in the right direction.
I think the -6 is fine for Saving Throws, Armor Class, To Hit modifiers, and Damage modifiers.
I still think the system is problematic for ability scores and maximum hit points.
1. Firstly ability scores and HP scale very differently. -1 to an ability score doesn't really do anything normally, you need a -2 to actually result in a change. It's even more drastic with HP where 6 HP is a relatively small amount nowadays. Max of -6 HP reduction is even less relevant for Monster HP (which scales differently even more).
2. Secondly, attributes are tricky because they do very little by themselves (other than the handful of tokens that have a minimum stat requirement). Most attributes have an impact on some of the other stats like Saving Throws, AC, To Hit, and Damage. So if an attribute gets reduced, which reduces AC/To Hit, does that also count as a "reduction" to AC/To Hit?
3. Also, does "reduction" include hypothetical abilities that set attributes to certain values? The traditional Feebleminde spell comes to mind, which sets Intelligence (and maybe Charisma) to 1. You could also imagine polymorph effects which set Strength or Dexterity to very low numbers.