This is a really good question. I heard BasicSwap uses CoinGecko which in turn uses CEXs best way may be to just turn off that feature entirely and let the order book decide. Though, Bitcoin has an extremely low chance of ever being delisted, so you could use bitcoin CEX ptice and let the order book determine how much BTC its worth (like this coin trades for 500k sats). then 500k sats is fiat price so display what 500k sats is worth as that coins price
Well I'm less concerned about being able to fetch a price and more concerned about DEXs being influenced by potential manipulation at CEXs. I think probably DEXs behaving like first class citizens and letting their order books float would be ideal, and letting any mismatches get arbitraged away.