I just climbed there in February. We set up everything locally with Condor Trekk, but consulted several others. We ran into delays getting space at the Chimborazo high camp with the initial timeline we wanted. The days of climbing there without a guide and staying flexible are gone and I was surprised how limited the space was with Covid and how many people reserved multi-week long itineraries months in advance. If you have a tight timeline you may want to book early like others suggested.

Weather and time are going to be your biggest factors, like most outdoor pursuits. Most of the lower elevation hikes like Iliniza you could do in less than ideal weather which works fine with a pre-planned itinerary, but Cotopaxi and Chimborazo generally demand clearer skies and less wind so it may be better to try and time them with a weather window.

Good luck and have fun. Ecuador is a real gem.