...and also fix the inflated AC issue.
Since redoing the animations for 4th, 5th, and presumably the coming 6th attack would be a lot of work, change the damage instead.
The issue is that the slower animations reduce damage, and back in the day an attack bonus was added instead to encourage people to not move to break the chain.
However to rationalize the increased attack, monsters ac was all jacked up, and in certain cases is still stupid high (A level 16 quest with a 60 ac monster is jacked).
So? Eliminate the added attack bonus for extra attacks entirely - or reduce it so that only the last attack gets a +5 say - and instead add DAMAGE for the extra attacks. And also reduce monster ac by 5 across the top for anything over 35-40 basically.
So the slower animation will be made up by increased damage if you go through the full chain, which will not only eliminate the DPS complaint/problem (and yes its a problem if as you go up in level you do LESS damage than you did the level before), but will help out and be a big incentive for DR monsters.
Easy fix, no new animations, and the initial intention of making players use all the attack animations will be fulfilled, without the current penalty - and AC will become more balanced to boot.