Check in /etc/fstab if you have the discard option for your SSD. Last time I checked, Manjaro does implement TRIM by default, but not this way.
They do not do the same thing. The timer does all the TRIM job once per week, and the discard option does the TRIM job much more continuously (it is issued each time files get deleted).
The timer way is arguably the most preferred way among the Linux community, and arguably the one preferred on many distributions, but I could not tell why Manjaro choose discard over following the trend.
It is useless to enable the timer if discard option is used. If you end up not using discard, well, enable the timer if you prefer using the timer.
And every time a file is modified. And there’s the problem. When the differences are too small, then the underlying storage medium may ignore the trim request. Using fstrim.timer instead ensures that there will be enough discarded blocks in the queue, and that the underlying device will effectively trim the blocks.
Another consideration is that the discard mount option reduces I/O performance. This too is why the fstrim.timer is preferred.
We had a bad batch of release candidate isos where it was disabled. It was supposed to be disabled only for the live system, but it got disabled for the installed version as well, along with snapd, tlp, pkgfile and so on. The issue is fixed with the next release which is scheduled to happen next weekend.