I think the excellent "full spectrum resistance" has a very good chapter on what it takes for someone to be radicalized, breaking it down into three needs, personal galvanizing experiences(experiencing racism, getting bonked on the head by a cop, etc)

radical catalysts(time and space to ponder things, analytical tools to examine experiences, intrinsic optimism that things can get better)

and action needs,
A community of people to take action with. Models for action. A mental narrative of how social change happens (or an imminent danger that demands immediate response). A sense that winning is possible, or perhaps that they have nothing left to lose. Biographical availability.

If they dont have these things, they can become embittered, soured, "curdled idealists" and not active in their pursuits.

I think if you can help a given person find these "radical triad" , encourage them to read analytical marxist books, help attend protests with a friend or other models of action, etc. than that can be a powerful way to create an active radical.

Kaiserreich's Left Kuomintang game-play is the gold standard against which all other HOI4 mods, demonstrably leftist or otherwise must be judged. It is engaging, reasonably ideologically coherent, uplifting(or not if you pursue one of the less savory paths), full of different choices that will impact the long term dynamics and where your new government ends up, tactically and strategically engaging with lots of different ways to engage in wars and alliances and perfectly utilizes the new HOI4 government power struggle mechanics.