Dhimmi bonuses are actually really strong especially for a rapid expansion game. If they're at high influence (which they'll usually have thanks to a high percentage of heathen land) you can pretty easily get that sweet -10% tech cost and +2 tolerance of heathens (or at least -7.5% and +1.5), which when combined with the +2 from the estate gets you up to +2 total with 100 legitimacy meaning you can ignore heathen provinces as soon as you start getting them. The manpower bonus privilege is also strong but comes with -5 loyalty which is a problem since the Dhimmi do have limited loyalty bonus options, but humanist ideas synergize perfectly with not converting heathen provinces and gives +10 loyalty so combined with supremacy over the crown, land rights, and calling diet you shouldn't have issues keeping them above 50% or even 60% loyalty. Obviously if you want to deus vult your way across the globe you can go ahead and just convert them since true faith unrest reduction is unbeatable but the Dhimmi are great, especially for Ottos or Timmies who want to blob like crazy.
tl;dr Dhimmi good, but you've got to keep a heathen province around for them to work.