You can simply reduce the size of the reserved space to 64 MiB. This will have an immediate effect on your free space for sda2, without resorting to resizing partitions or reformatting anything.
sudo tune2fs -r 16384 /dev/sda2
By default, a newly formatted Ext4 filesystem will reserve 5% space, which in your case is a waste of almost 1 GiB.
Other culprits include the use of Timeshift on the same filesystem, among other factors mentioned in this thread, such as your temporary files, pacman cache, and journals.
Using Timeshift on the same filesystem is the single biggest factor, however. 
The number 16384 is for 16384 “blocks”. Your blocksize is very likely 4096-bytes, so 16384 blocks multiplied by 4096 bytes equals 67108864 bytes, which equals 64 MiB.