@Winnie. Ok, i will also try your rules, since now i have still very rare timeouts. The system is now already running much better with kernel 5.10. I assume that you mean 60-ioschedulers.rules and not 60-ioscheduler.rules. Note that KDEneon, which runs without issues, uses discard in fstab!
I have another system (HP m8000) where for Manjaro XFCE fstab is also configured with discard for the Samsung SSD 860 EVO 250GB (/ and swap). Seems that this was default at installation time, some years ago.
timer and service info:
● fstrim.timer - Discard unused blocks once a week
Loaded: loaded (/usr/lib/systemd/system/fstrim.timer; enabled; vendor preset: disabled)
Active: active (waiting) since Tue 2021-07-20 19:38:02 CEST; 13min ago
Trigger: Mon 2021-07-26 01:14:03 CEST; 5 days left
Triggers: ● fstrim.service
jul 20 19:38:02 gsm-man-kde systemd: Started Discard unused blocks once a week.
○ fstrim.service - Discard unused blocks on filesystems from /etc/fstab
Loaded: loaded (/usr/lib/systemd/system/fstrim.service; static)
Active: inactive (dead)
TriggeredBy: ● fstrim.timer
Edit: Sorry to say, but your rules appear to be not good for the BX500: Frequently freezes with timeout. So bfq seems to be the best.