The fact that he has Perfect Sage Mode markings means he can mix the Senjutsu chakra with his own perfectly, so since he can't maintain it, that indicates that his chakra pool is not large enough, which means fusing with Fukasaku and Shima would do nothing.
Jiraiya = Chakra Pool but no Control
Minato = Control but no Chakra Pool
Lol no, nothing to do with chakra pool, get that crap outta here. You have no idea how long he can even maintain it. He said he can't maintain it "long". Whether that is 2 minutes or 5 minutes, it doesn't matter. Who told you minato doesn't have the chakra pool, that's garbage. Unless you possess "enormous chakra", you cannot draw in nature energy. It's nothing to do with his chakra, it's just lack of practice. Naruto trained to extend his sm, it was only 5 mins at first. Meanwhile Jiraha with sage toads lasted way longer than 5 mins. So you are saying Jiraya has more chakra than Naruto since he can make it or longer,

.
Amphibian technique maintains combat readiness. Roles are divided, the toads gather the nature energy while the other can move. The toads maintain sm, not minato.