Anyone with experience with using Just War?

I've been having fun with using Religion, but so far I've been mostly doing it more peacefully via Swords into Ploughshare and spreading by Religious Text/Itinerant Preacher. I've been brainstorming something along the line of a massive Religious Empire from Liberty + Pagoda, and couple that with Holy Warrior and Just War. Near mid-game I can pump out free units every few turns with Holy Warrior and enhance my sieging ability by sending a missionary ahead of me. Maybe grabbing Evangelism if I can afford to detour to Piety to help in attacking cities where another Religion has already settled.

I'm thinking about using Boudicca due to her insane early Faith gain from her UA and Pictish Warrior, and although the faith gain doesn't carry over as a promotion, having strong Pikeman with bonus in fighting in foreign land still helps. of course.

Yeah. You need to send a GP as part of your invasion force (or maybe a missionary if its early). You cant do pagodas, and holy warriors, and just war--too many things vying for your faith. Have to pick. The good news is, once you take the city, its already your religion.

Yeah I was playing around with it just now, and like you said, too many things vying for my attention. Right now I'm not sure whether I want to go a Liberty + Piety hybrid, or just straight Piety for the full religious conquest theme. If I go straight Piety, then Ancestor Worship + Asceticism could make a quick Shrine first build a decent choice for the immediate Faith + Culture + Happiness boost.

Just war works pretty well. Attack a city with Just War, SoZ, & Siege promotion that's an 85% bonus who needs ranged units when ya got Just War

As Celts? I like Honor. Tradition first, get the Picts out (meaning hurry to Bronze Working), choose Honor opener, farm barbs, then choose Discipline and Warrior Code. Milk that faith. It's not the most OP strategy out there, but you're playing the civ true to its character. You should be able to get that second GP (and pick Just War) with no problem. You picked Just War, you intend to warmonger. Go Honor.