I am hardly an expert on Cinnamon or Linux power management, but I’ll offer some ideas.
(Although not my primary computer, I do have one up-to-date installation of EndeavourOS Cinnamon on a 9-year-old computer, and I do see the Suspend button.)
You have a very powerful, modern system, so limited system resources and functionality are unlikely to be the cause of the problem.
What is the result of this command?
cat /sys/power/mem_sleep
On my system I see
s2idle [deep]
which, I think, means essentially the same thing as S3. In other words, I think, my BIOS has advertised to the OS that it supports the S3 power management mode, which includes support for these functionalities: Suspend (to RAM), Hibernate, and Hybrid Sleep.
What is the result of this command?
cat /etc/systemd/logind.conf | grep -i suspend
On my system I see
HandleSuspendKey=ignore
#HandleLidSwitchExternalPower=suspend
#SuspendKeyIgnoreInhibited=no
Nothing here prevents Suspend from being offered on my system.
I know nothing about TongFang motherboards, but I see in online screenshots that the TongFang BIOS may include a setting for “S3/Modern Standby Support.” If you change this setting and reboot, does the Suspend button appear?