Is it permissible to combine and shorten the Maghrib and Isha prayers half an hour after the Maghrib call to prayer, upon entering the outskirts of Riyadh while traveling to Mecca?

What you did is the Sunnah. The Sharia has permitted a traveler — if their journey is permissible — for a distance that allows for shortening prayers (approximately 83 km) to combine Dhuhr and Asr, and Maghrib and Isha, either as a forward combination (Jama' Taqdim) or a delayed combination (Jama' Ta'khir). They may also shorten the four-rak'ah prayers to two rak'ahs. The Prophet, peace and blessings be upon him, did this during the Battle of Tabuk.