Historical Project Mod Plus (HPMP) is a stand-alone update built upon HPM 0.4.6.3, building upon it with both bug fixes/QoL improvements, and an Expansion of several HPM mechanics I really liked, but felt underdeveloped. Many improvements towards the late game.
Slavery has the advantage of allowing early/quick industrialization, but it can be risky, as all those slaves will not become craftsmen upon becoming free, and if your industry relies on extensive slave labor, it can be a crippling change.