I believe the seal is A mean to bypass the requirement for big chakra reserve,
The chakra in her seal comes from her own reserves. Keep in mind that when Fukasaku was describing the gathering process for sage energy, a diagram was made showing how natural energy is mixed into the chakra moulding process. Hence the development of a more powerful type of chakra. Sage Mode is the coalescence of physical stamina, spiritual energy, and natural energy. You're mixing those three fundamental ingredients together.
Given that Sakura's seal contains premixed stamina and energy, without nature's force having been added, it would logically be unusable. Think of it how you would mixing colours together. You want to create black but you only mixed blue and red together. Instead of black, you made purple.
and if that true than sakura ssm is her triumph card.
Sakura described the seal as being her "true power". It stands to reason that it was the pinnacle of what she could attain. Logically it would be since the Yin Seal is the primary indicator of a master healer.
And it will make sakura high tier kage level.
Only if you're using that term loosely. If she did acquire Sage Mode, it would be limited to short bursts since she doesn't have a fusion partner to gather energy for her. Leaving her critically vulnerable in combat. Not only that, having increased physical stats doesn't mean that she will even become the equal of SM Naruto or MS Sasuke. There's more to combat status and ability aside from brute strength or speed.