If you build cathedrals in advance of converting and keep missionaries going full time you can convert 500+ provinces yourself still, on 4 missionaries (assuming you go revolutionary).
Subjects that don't have positive tolerance of the religion in a province will convert. If you build cathedrals, even non-religious subjects contribute decently. Client states will work if you make them republics (no tolerance from legitimacy, will convert), but nations that take religious are better.
revolutionary cb is great for that. you can force vassalise big nations and force religion at once sometimes.
It is also the only CB in the game that can force religion on heathens in peace deals, which allows you to emergency bypass religious zeal.
Most Somali nations release with religious. Miao does in China, while Shun will convert since it doesn't take humanist. Malacca can handle Indonesia, same with Brunei. Mutapa takes religious for that group. West Africa only really has Songhai, but you can just convert that.
India similarly has few that take religious (Vijay will), so if not using Vijay then pick a Hindu that doesn't have +50% core cost or +3 heathen tolerance like Gwalior or a South Indian like Madurai/Venad/Kochin...or just use Catholic client states there, make room for cathedrals before giving them land and then build them so they get ~7-9% conversion strength on accepted culture (2 base, 2-4 from decisions, 3 from cathedral). For North India you can use Nepal.