You just said the tip yourself, use a third party tool/plugin to make things easier.
That is what VRCQuestTools is.
Best size saver is to downscale resolution on your textures,
set mesh compression, although I don't recommend that cause it makes your mesh look like shit
remove gogoloco, or other excessive animations you don't need.
Remove parts of prefabs/gameobjects that use lots of size.
You could try using a script like Build Size Viewer to help show which files are bigger in size and which ones are using less on your build.
(https://github.com/MunifiSense/VRChat-Build-Size-Viewer)
Usually my recommendation to any one trying to get into Android conversion is: Don't.
