Turn-Based Tactical RPG

I’ve been brainstorming an RPG for some time now, and want to start working on making it. It is very loosely based on the DnD3.5/Pathfinder 1e system, though I’ve modified it a lot to the point I don’t think it’s all that similar anymore. The type of combat system I’m imagining for it would be akin to what is in Temple of Elemental Evil (with better UI, though), or the turn-based modes for Pillars of Eternity II and Pathfinder Kingmaker.

Is there already a template for something like that, or would I need to design it from scratch? I want to have the player enter combat upon engaging enemies without changing maps (unless on the overworld, perhaps).