Free, natural-sounding TTS for a website voice assistant? (Browser voices sound too robotic)

I’m building a voice assistant for my website. Client click “Call” and talk to an AI that answers questions about that page(its a /help route page).

Current setup

• Backend: FastAPI + Groq LLM (resume parsed once at startup, short spoken-style answers)
• Frontend: plain HTML/JS using the Web Speech API (SpeechRecognition for the mic, speechSynthesis for the reply)
• It works, but the browser voice sounds very robotic

What I need

• A free (or generous free tier) TTS that sounds natural
• Feels like actual call (eg. sarvam ai paid call ai)
• Good Indian English accent, with Hindi/Hinglish as a bonus
• Low latency, since it’s a live conversation
• Usable from a FastAPI backend or directly in the browser

What I’ve found so far

• Edge “Natural” voices (free, but only for Edge users)
• Sarvam Bulbul (great for Indian languages, but only ₹100 of free credits)
• edge-tts (free, but unofficial)
• Piper and Kokoro (open source, but I’m not sure about hosting them cheaply)