To be flat out honest,
The Spirit Bomb should be Baseline just like Infinite Zamasu. It's heavily implied that the amount of power that was in that Spirit Bomb would of worked on any villain that they faced, looking at the line up that's Merged Zamasu/IZ, Goku Black, Hit and U6 Warriors, and Golden Frieza. This makes reasonable sense too, It had power given from many top tier Super characters on the Dragon Team, notably Gohan, 17 and Frieza donated Genki to the Spirit Bomb. The Spirit Bomb, and Spirit Energy has even been shown to be even more powerful than fusion in the show times before, example Trunks Spirit Sword in the arc prior had enough energy to slice Zamasu into 2.
If the Spirit Bomb is Low 2-C, Who would get upgrades?:
SSJ Kefla (Should have an At least 3-A to Low 2-C)
If the Spirit Bomb was Low 2-C then Shin's and Whis' statements towards Jiren being the strongest have more weight. SSJ Kefla would also be rightfully Low 2-C since she bodied SSJBKKx20 Goku with a single kick. Her power was rising drastically and Whis stated that she even surpassed the Spirit Bomb. I think Whis using Goku's Spirit Bomb as a benchmark even solidifies the point that it deserves a Baseline rating.
SSJB Vegeta's Final Flash (SSJB Vegeta should be at Least 3-A, Low 2-C via Final Flash)
Both Goku who threw the Spirit Bomb and Belmod directly stated that his Final Flash has more power then the Spirit Bomb and there can even be a case where it even surpassed Omen 2 Goku since Belmod was shocked that he stated, "This is the power of a Saiyan?" Goku having fought Jiren before is even shocked that Vegeta was able to read Jiren's attacks, but if Jiren was holding back so much against Vegeta that he used less power than his initial fight with Goku, why would Goku be shocked that he was able to land a hit, he can read Jiren's power just fine and he's the one that tells us when Jiren displayed a hint of his true power.
Golden Frieza(For now an At least 3-A key is just fine for Frieza)
Not much to say for Frieza, he did eat Toppo's Hakai dead on and handled his own against Jiren decently albeit a weaker Jiren. I am fine with where he's at though we could make it At Least 3-A because of how much above baseline both Toppo and Jiren are.
17( 17 is 3-A but his Barriers should be Low 2-C)
This man has been through it all, he managed to help defeat Anilaza by figuring out his weak point and made past his blast of energy, funny enough when 17 smashed into Anilaza's head he actually put his barrier around his hand for his punch, maybe that could support his barriers have that sort of power. He took hits from GoD Toppo and took attacks that were even meant for SSJBE Vegeta from Jiren. NOT ADVOCATING HE'S ON THAT LEVEL, but theres obviously more to 17. Again his barrier and "Self Destruction" held back and stopped a Low 2-C attack.
And Anilaza, which I've explained above. Again a lot of the refutes against Anilaza back then were used against Kelfa and those were wrong.
so what's the TL;DR of all this?
'The Spirit Bomb should be Possibly Low 2-C' as Shin and Whis used it to judge Jiren's extremely casual power to be above IZ. In return all those who have feats and statements that would scale them higher than that Spirit Bomb should be Low 2-C which really isn't a lot of people. Those who have the potential to scale higher but don't have enough evidence (Goldey, Goheezy, Better than Apple) should have the at least 3-A rating for being able to hold their own against Low 2-Cs that scale way above Baseline.