Feels like going with unity per # blockers isnt really worth it - unless we get some rework of planetary blockers/features, so they can regenerate somehow. Like it feels a bit pointless if you take someone's world and they have already cleared the blockers there.
If the opponent is clearing their blockers by the time you're conquering them, that's a good thing- it means the opponent is either being very inefficient with their early-game teching (blocker techs instead of useful ones) and econ, or is so desperate that they had to commit to blocker clearing despite the inefficiency, both of which are probably why you're the one conquering them and not the other way around. This is a bit of the inverse of the criminal syndicate dynamic: you're not beating the syndicate if you spam early-game enforcers, because while you're doing that it's still growing.
Environmentalism as a civic is for early-game wide expanders and pop bloomers, especially xenophobes, to have an aggressive tradition growth without having to commit the pops towards culture workers instead of scientists. It's closer to a xenophobe war economy set-up for a year 25 war, where you spend your first tradition as an econ base (prosperity after dipping into exploration's tech draw chance) leading to Supremacy as a second tradition.
Mechanically, it's a civic that rewards/incentivizes you to invest more heavily in your mining/mineral capacity early on by letting you turn those miners into unity (the pop-free unity that comes after the 1000 mineral commitment), before transitioning that mineral over-capacity into industrial districts and CG and then alloys. Even if most planets 'only' provide an average of 6-8 unity via the imposed and natural blockers, this is basically an extra politician per world at a point in the game where politicians are the core of one's unity economy, only without needing the pop to be the unity job. Early traditions are functional techs, and expanding unity production before science production is quite viable- and this civic allows you to expand the unity production without actually taking the pops from being science.
This isn't as much a unity infusion as, say, Parliamentary System, but it is one of the only civics that actually adds pop-free unity into the early game economy, where it's most useful.