Why can't i just change time speeds in percentages like I need to?

Premiere's speed for time interpolation is very nice. Ae is different - it gives you finer control but may seem less optimal for ramp changes in speed (because the clip does not automatically compensate for the change like in Pr).

you can use the Timewarp effect in Ae and you will get speed parameter same as premiere's and finer control of the interpolation between frames.

3rd party tools you may consider:

https://www.proloststore.com/products/speedramp
https://revisionfx.com/products/twixtor/

Time remapping can be precisely accurate if you learn how to work with the Graph Editor or just do a little math. The value graph gives you seconds and the speed graph gives you seconds/second. Here is an example that plays the first section at 2 seconds/second (double speed exactly) then takes precisely 25 frames to slow down to realtime of 1 second/second. You can't do that in Premiere Pro, not even close. Hovering over a keyframe gives you specifics.

If you don't want to use Time Remapping then clicking on the third icon from the bottom left in the timeline gives you the same, if not better controls that you have in Premiere Pro that will allow you to stretch, set duration, or precisely change the in and out point time of a clip.

gives you:

But personally, I think it's a lot easier to control the timing of a clip using Time Remapping and just paying attention to where you put the keyframes and how you move them. I'd say you have much greater control in AE than you do in Premiere Pro or any other NLE i have worked with. You just have to figure out how to understand what you are looking at. Most folks have a very hard time visualizing what is happening with time when they start bending it.

Right click on your layer > Time > Time stretch > enter a percentage in the Stretch factor field.