I don´t see how you can maintain a fleet admiral agenda if you don´t even think Sengoku is>Garp. If the fleet admiral isn´t inherently above all other Marines, Kuzan is clearly not much weaker than Sakazuki, the man who refused to even serve under him, and had Blackbeard shitting himself as hard as Sakazuki did. You could try excluding the other 3 and argue that there are 2 FA marines, but Sakazuki literally doesn´t even believe it to be his place to criticize Kizaru, and Kizaru doesn´t see Sakazuki as his superior beyond officially.
You could argue Fujitora and Aramaki are the exceptions, but you´d honestly be bsing to say Kong has even looked as good as them, let alone as good as Sakazuki and Kuzan, just when it comes to sheer presence. There also isn´t much to suggest Fuji and Ryo are that much weaker than Sakazuki, at most a sub tier.