Even if she had a fanfic SM shed be over a tier below them at that point still regardless
I genuinely dont think it can be done short of making her a perfect Jin of one of the lower Biju honestly.
You need a powerup on that level as thats the level she’s trying to compete with and she cones nowhere near it in canon.
Her own novel has her below a pseudo V2 Jin level stated to be below real V2 Jins...She literally cant even damage said opponent and instead needed to effectively poison him with a medical ninjutsu cheese to lower his durability.
Theres a monumental gap between being unable to breach V2 Jin protection and EMS/BM tiers
Maybe she could learn Bringer of Darkness, as Tsunade is senju she may find some scrolls to teach her.
She should also learn some elemental attack based on her affinity. She needs some long range jutsu.
Improve her shunshin.
That are thing that would make sense in manga. But you could potencially give her all bunch of thing:
-learn toad SM
-implamnt byakugan from someone
-learn multi shadow clone
-make summoning contract with toads, snakes, birds, dragons, vampires...
-learn SM byakugo amped yomi numa
-learn raiton armor
-seal other 7 jins inside her
-learn 8 gates
-insert hashirama boob (so she could have some)
-give her gunbai
-give her samaheada
and so one..
-a proper, grand-scale acid bullet ninjutsu
-ginga girigiri dai panchi
-a special club/staff weapon that can store her chakra and fire off -Btooms!!- on their lvl -a kienzan no jutsu advancement of mesu tech
-many hidden slug hands
She's factually and canonically already on their level.. you're discussing a non existent fanfic version otherwise, but feel free to do so.
If even the kaguya feats aren't enough, then I don't see what can be done really
It's already a miracle that she slightly surpassed Tsunade, anything more is asking for the impossible
But seriously, considering Sakura's attitudes&skills i would say the most natural power ups for her that also don't ruin the feel of her character, would be:
- Genjutsu, both individual Genjutsus with high potency and large AoE ones.
She is a master at fine chakra control and had Genjutsu attitude from P1, although she never developed it and focused exclusively on medical.
So it seems a natural power up for her if she went and actually trained it;
- slug sage mode (Hashi Sage Mode).
Again fine chakra control skills and also fits perfectly with the strength&physicality fighting style she has.
Her strength would explode with this;
These are the best and most narratively consistent power ups you can give her imo.
Maybe you could add some mid ranged acid Jutsu, poison hax, and similar.
But the "feel" of her character will start to get ruined by adding more and more stuff that is increasingly more distant from the strength, regen & medical tier chakra-control themes.