I like playing Tall too and I sympathize with the idea of making Ascensionist a generic Civic as opposed to being a Spiritualist one. Yet I've come to find that 45% increase via Harmony + Holy Covenant is more than adequate. I mean sure another 25% is massive with respect to Tier 10 output(362.5% vs 425% or 62.5% difference), as is the Empire Size reduction(72.5% vs. 85%). Yet if your goal is to minimize Empire Size, you'll come to find you don't really need it. Pops can be completely removed outright with Under One Rule Origin -> Civil War outcome with Pacifist Harmonized Society Modifier -> Shift to Fanatic Pacifist + Egalitarian with Beacon of Liberty -> Cybernetically Ascend to get Democratic Concurrency and tack on Harmony + Domination + Psionic Theory for 100% reduction. You can optionally add Balance in the Middle(Tier 4 Greater Good) for another 10% reduction(which if you're able to pull off will permit you to forego having Domination Tradition). I'm hopeful that 4.0 will add a Genetic Ascension alternative.
Similarly to Pops, Districts can also be outright removed but I think there is a bug with Empire Size via Planetary Ascension since I don't think it factors in buffs from Harmony, Ascensionist or Holy Covenant. If it did, then all you need is Harmony Tradition finished, Tier 10 Colony(Tier 9 with Holy Covenant) and a Governor with Urbanist Destiny Trait. Both Planet Ascension and Urbanist Destiny Trait share the same Colony Scope modifier and hence additively stack so 37.5%(Urbanist Sector bonus) + 62.5%(Tier 10 Planet w/ Harmony bonus) = -100% Empire Size from Districts. With Holy Covenant, this should be achieved at Tier 9.
That just leaves Systems(which Ascension doesn't affect) and Colonies(which is already at -75% via Expansion + Imperial Prerogative). Assuming you have Harmony + Holy Covenant, the difference between having Ascensionist and not having it is basically ~3% or 0.3 Empire Size. Unless you have like 200 Colonies(which would take eons to Ascend), that's peanuts in the grand scheme of things.
The only thing where I wish I had Ascensionist is the cost of Ascending planets. That 10% really becomes impactful when you've ascended like 10-15 colonies to Tier 10 and the Unity cost starts pushing into the Millions and hence 10% saves you gobs of Unity. That is pretty much the only thing I miss and I honestly would prefer there be more Tradition sources besides Harmony(Prosperity would be an ideal candidate).
