I looked up how to do this and the tutorials use the time stretching option under output properties, but this value cannot be keyframed meaning that I can’t slow it down and speed it up mid animation. Is there any way to do this?
You can put the keyframes closer or further apart from each other. Closer makes it faster, further makes it slower.
Wherever your frame is at, (Ex: in the photo below my frame is at 21) you’re able to scale selected Keyframes away or closer to the 21.
Adding more frames makes the slow-mo smoother.
Did you have a different method of speed ups / slow downs in mind? If so, describe it to me.
It would be convenient to use an editing program for these tasks.
For a blender, see the video that explains how to edit the NLA.
Directions such as fast playback → slow motion → fast playback → reverse playback, etc., which can be seen in some videos, are created using editing programs.
It will be helpful if you refer to the video.
Information about the frame is also helpful for the work of the blender.
I know I can do this but it only works for simpler animations and gets very tedious once there are multiple animations or if I want to tweak an animation or something so that’s why I’m looking for something like the time stretching option that does it automatically
I’m trying to keep it all in one render and I don’t want to use external programs. I think for me to edit in Blender’s built in video editor, I need to render the animation first and then import it into the editor which is basically no different from me just using any other external video editor
Yes, that’s right.
It’s just a little uncomfortable. 
If you need a free program, use DaVinciResolve.
Editing is a common way of working and can reduce stress.
I use Premiere Pro and After Effects and other programs in other projects, but I don’t want to use them here and it would take a lot more work and rendering time I think if I decided to use them here because of the specific animation I’m working on
Either, render at a higher FPS or you stretch out you animation say from 100 frames to 1000 frames. There is also the option to use time scaling in render properties. Either of the 3 ways you will end up having to render lots and lots of frames, and choose your playback properly when you PNG seq → video
Using the NLA editor worked out fine for me