I am still a newbie to all this so forgive me if this is all munbo jumbo.

So, your issue is having a fixed speed animation for different animations with a varying ammount of keyframes, right?
What you do with nextframe is steping from one keyframe to the next directly without bothering with percentages, is that right?

So you could also just put a number in the percentage calculation that makes a fixed percentage for all animations be diferent for each animation

So for example: animP += speed * animation_length_factor * time_step;

You would have to store in memory each of the animations length, but you would still have speed as controling all animations speed, but then animation length influencing each particular speeds.

Sorry if this is all garbage, just ignore.