For what its worth, adding my 2p.
I only target a pH 0.8 drop, if I push it to a full pH 1.0 drop my Amano shrimp go nuts around the top of the tank, and I lose cherry shrimp. My theory is that my atmospheric CO2 levels are higher than that used to determine the original pH 1.0 drop recommendations (Nottingham/Midlands is known for poorer air quality than a lot of the UK) - as mentioned in the thread that
@jaypeecee linked to. The case might be the same for you being in Greater London, so I think your pH 1.2/1.3 drop is far too much.
I have the same drop checker as you, and it's a good unit - far easier to read than the clear glass ones. That said it's hard to tell from your photo as its too dark, but it looks like you're on the upper end. A full tank shot would also be beneficial.. It would be good to see a fresh photo well lit up so its easier to see the drop checker colour. This is mine at my 0.8 Ph drop:
Also bear in mind that the drop checker is only telling you the CO2 levels at that position in the tank. It could well be, if your distribution isn't that great, that the levels are much higher elsewhere in the tank. I personally use two of these drop checkers - one where I estimate the CO2 is likely to be at its lowest (the end of the in tank circulation) and one at the opposite end of the tank where I expect 'average' levels to be - in direct flow of the CO2 source. If distribution is optimised, they should be the same colour. In your case you could try moving the drop checker elsewhere in the tank, to see if the CO2 levels are different (bearing in mind it can take 2 hours to change colour).
Given you are using spray bars, I think you should really look to add an inline diffuser or better still an external reactor (they are easy enough to DIY). That will help you to achieve better and more even distribution of CO2 as the output of both of your spray bars will all have the same CO2 saturation - I think the single point source in-tank diffusers only really work well when using a single point source filter outlet that is responsible for the entire tank distribution.
Finally you mention minimising surface agitation - I think this is a mistake, as you want to tray and maximise dissolved oxygen levels. Yes you may chew through more CO2 as a result, but it will help your livestock, especially if you have a deep tank - I also believe higher DO levels help minimise algae.