KCM works by taking a chunk of Kurama's chakra and then using it as your own. The user doesn't really touch their own chakra reserves much, so they only get a boost in chakra capacity.
Senjutsu works by absorbing enough natural energy to form a balance with your own chakra. The fusion of Natural Energy and Chakra produces Senjutsu Chakra.
The biggest risk with FBS + Sage Mode is this balance. Natural Energy is overbearing, its inherently more powerful than regular chakra and if it becomes too large it'll overpower the user's chakra and petrify them.
Taiyo needs to maintain this balance
while FBS is burning through his chakra, which means he needs to expend Natural Energy just as fast as normal chakra, else the Natural energy will devour his own chakra and kill him.