Imagine again my friend ! 🙂 Indeed I did cut every single one of those 3 way junctions myself (I was having issues with radial symmetry at the time, which would have saved me loads of it ideally) but I am an extraordinarily patient little modeller, so went to the zen zone and got it done in 10 mins. Thereafter I could use Loop Cut (re-use cuts, 50%) to bridge each diagonal flow from top to bottom (leaving each end unconnected), and then remove ngons command to connect those to the custom topology in one hit. Whole thing took 15 mins, which I consider... worth it !
With the joy of hindsight what I should have done there was earlier in the modelling process, when I subdivided just the non-diamond form topology. Had I done it all at the same time, I could have avoided doing it manually later, but I didn't, at the time, because even non smoothed L1 subdivision was disrupting the straightness of my topology. Sometimes, 'manual' is the only way to go !
CBR