I agree with the majority of it.
Atm i'll say Sakazuki is the 4th strongest person alive.
Excluding Linlin, Kaido and Dragon since I really have not a shred of an idea of how strong they are, so the bit about him being able to beat Linlin and Kaido, i'm not sure about. Plus I think Teach is on the same level as him but slightly stronger. Same for Kuzan, they have been hinted at being pretty equal and therefore, on the same level when it comes to strength and power, although I do think Sakazuki is a tiny bit stronger.
But I agree, he is the strongest person in the Marines atm, from the way he handled Newgate I think it's safe to say he could defeat Garp and Sengoku, even if by barely.