Developing high-efficiency solution-processable thermally activated delayed-fluorescence (TADF) emitters, especially in longer wavelength regions, is a formidable challenge. Three red TADF emitters, namely NAI_R1, NAI_R2, and NAI_R3, are developed by phenyl encapsulation and tert-butyl substitution on a prototypical 1,8-naphthalimide-acridine hybrid. This design strategy not only grants these molecules high solubility, excellent thermal stability, and good film-forming ability, but also pulls down their charge-transfer (CT) energy levels excited states. Furthermore, dispersing these emitters into two different host materials of mCP and mCPCN finely tailors their CT-state energy levels. More importantly, a synergistic combination of molecular engineering and host selection can effectively manipulate the competition between the radiative and nonradiative decay rates of the CT singlet states of these emitters and the reverse intersystem crossing from their triplet to singlet states. Consequently, the optimal combination of NAI_R3 emitter and mCP host successfully results in a state-of-the-art external quantum efficiency (EQE) of 22.5% for solution-processed red TADF organic light-emitting diodes (OLEDs) with an emission peak above 620 nm. This finding demonstrates that a synergistic strategy of molecular engineering and host selection with TADF emitters could provide a new pathway for developing efficient solution-processable TADF systems.