The changes in physical, chemical, and gas permeation properties of a heat-treated hydroxyl-functionalized Tröger's base-derived intrinsically microporous polyimide, 6FDA-HTB, were systematically studied. Polyimide samples were heat-treated for 30 min at a fixed temperature to form (i) polybenzoxazole (PBO) by thermal rearrangement (TR) from ∼420 to ∼440 °C, (ii) polymer-carbon transition (PCT) from ∼460 to ∼500 °C, and (iii) early-stage carbon (ESC) membranes from >500 to 600 °C. As frequently observed in previous studies, TR-derived PBO membranes showed an increase in gas permeability with a concomitant decrease in gas-pair selectivity. The intermediate PCT region represents a transition state from a PBO to a partially carbonized polymer with superior properties by showing a simultaneous boost in gas permeability and gas-pair selectivity. The ESC membranes formed between 550 and 600 °C possessed further enhanced selectivity due to tightening of the amorphous CMS structure. A 135-day aged 6FDA-HTB-600 membrane showed very high hydrogen permeability of 3544 barrer coupled with an outstanding H₂/CH₄ selectivity of 939. The mixed-gas separation properties of selected aged samples, pristine 6FDA-HTB, PBO (6FDA-HTB-420), intermediate PCT (6FDA-HTB-480), and early stage CMS analogs (6FDA-HTB-550 and 600), were investigated using a 1:1 CO₂/CH₄ feed up to 30 bar. In this series, early-stage CMS membranes displayed the best mixed-gas CO₂/CH₄ separation properties with performance significantly surpassing the 2018 polymer mixed-gas upper bound.