Too many words, too few numbers for me. I seem to get lost in the discussion LOL
I definitely misread him. He said mixing 2 to 3 themes for composing your strategy. And having up to 12 themes (i think he wrote somewhere about 20 themes; this i think would be way too much and this lead me to thinking he meant atomic functional themes/branches). I do not know how many tiers and techs he thinks of.
I think the current self-sufficient theme suggestion from Oberlus is less than 300 techs organized in 6 tiers in 5 themes (I think 300 techs is too much).
My personal preference would be not more tech than fits on a single screen (seems I am the only one?).
Another variant of lean themes: Lets say you organized your X techs in T functional themes. Now see if you can remix the techs into about T sparse self-sufficient strategies (self-sufficient in the sense that you could win a game under some circumstances; e.g. hulls with many externals and good armour parts; and sparse in the sense that taking tech from other will be necessary to win in many cases: lousy weapons and barely sufficient supply). Then fluff the themes up (find a fitting remembrable story for the theme) and harden them afterwards (minor tweaks to the techs to fit the story better as long is this does not hurt composability).
Another variant of self-sufficient themes: Lets say you organized your techs in self-sufficient themes which each basically support a single way of playing the game. If you make unlocking tiers and researching in other themes kind of cheap (because six is quite many) you basically have access to all the tech and are not railroaded into your "first" theme.
Once again the main benefit I see for the tier system as opposed to any tree based system is that the UI gets way more structured and can be much more effective screen-space wise while still offering a great overview.