You should easily be able to get to T5 from St Pancras International in just over an hour, taking the Piccadilly line direct from Kings Cross / St Pancras to LHR T5. As alternatives, you could use a combination of the tube and then either the Elizabeth Line or Heathrow Express, but these will cost slightly more and offer no considerable time saving from where you begin your journey. Assuming your Eurostar is on time, this will get you to T5 in plenty of time. One thing to keep an eye on is that engineering works causing disruption seem to have been quite commonplace on the Piccadilly line lately, so do check ahead of your journey.

From recent experience, you shouldn't have to be at Gare du Nord for your Eurostar check-in much more than an hour ahead, even on a basic standard class ticket. Eurostar usually email the day before with the recommended check-in time.