Consider it a loss and play it out for instructional value.
“Should I start spamming subs everywhere? Or would it better to ignore the large fleets and simply spam infantry all over the mainland?”
It’s good to ask specific questions. But it may aid development as a player if you can come up with answers to those yourself. When you do something specifically, what is your opponent’s specific counter? What are the general problems on the board? How could those general problems have been avoided?
“gut instinct”
Developing that instinct is good. But being able to explain your reasons is better. There are a number of specific mechanisms that lead to probable Axis loss in this game, assuming your opponent plays optimally.
(However, we also know the opponent isn’t playing optimally. And how do we know this?)
(Edit - excellent screenshot by the way, could view the specific numbers of each of the units.)