1) Your PVI-3.6-TL-OUTD Inverter is larger than it needs to be for your present setup.
2) If your Romag panels are Vmpp 30.6V per panel > 7 East facing = 214.2 V, 2 East + 4 South = 183.6V.
3) The voltages on each of your strings are operational but not optimum.
4) Putting 2 East with 4 South will reduce the amount you can generate on the 2nd string.
5) It would be benefitial to have 9 East facing panels connected to 1 string, but what would you do with the 4 south facing panels you've currently got, their voltage wouldnt be high enough for the 2nd string on a PVI-3.6-TL-OUTD ?
6) Aurora Optimisers may be benefitial, but at what price. What will you gain for your investment?
7) PVGIS indicates 9 East facing 235W panels in Durham (2.115 KWh) could generate 1400 KWh.
PVGIS Indicates 4 South facing panels 235W in Durham (0.94 KWh) could generate 784 KWh.
Thus 2184 KWh is the likely theoretical amount you'd expect to generate with a properly designed system.
8) Solar system design isn't always straightforward because of the roof types, spaces, orientations & inclinations installers have to work with.
9) Do I think the installer designed your system to its optimum, no?
10) How much could you gain by altering this system? 2184 KWh V 1815 KWh (17%)?
11) In reality solar optimisers are unlikely to generate what you'd expect because of the low combined south facing panel voltage, shading issues, greater efficiency losses etc.
If you had more roof space to install more south facing panels, you could seperate your East facing panels from your south onto individual strings with higher combined voltages that would improve things.
It may be best for you to accept what your generation is and be pragmatic about it.
Many people may generate more than you but how much of it do they productively use 33%?
How can you better utilise the energy you generate to exceed typical usage?