"And they're surprisingly low pressure, fuel goes up to 45 bar, oxidiser only up to 30 bar."
What you've missed is that each of those pairs of turbo pumps operates in series.
What you've listed is the output pressure from the Low Pressure Fuel & Oxdizer TP's.
The "High Pressure" TP's are driven by the flow from the Preburner and are more like 7000psi (around 470bar) (to feed the Preburner) feeding the Main Combustion Chamber operating with a back pressure around 2-3000psi.
Given that studies of engine costs suggest that development costs scale as the cube of maximum chamber pressure this may explain why SSME was such an expensive engine (it's also pretty big and uses liquid Hydrogen, neither of which help).