Web-apps which call speechSynthesis.getVoices() used to get a couple of good quality TTS voices which are still available in the iOS 18 simulator: com.apple.voice.compact.es-ES.Monica and com.apple.voice.compact.es-MX.Paulina
Unfortunately on a real iPhone running iOS 18, these voices are not available, which is a regression since iOS 17 had them. The only Spanish voices start with the prefix com.apple.eloquence.es and are of worse quality.