Does anyone know which wallets have Silent Payments/BIP-352 already implemented in their UI
There are none.
I believe it could be such a useful tool for groups of people that require more privacy and anonymity just like the Truckers' Protest, to avoid govenment sanctions.
Absolutely. But BIP47 or hosting a payment server that generated a new address for each deposit would have achieved the same result.
If BIP-352/Silent Payments was implemented, and the Trucker's Protest used it to receive donations, then decided to send those outputs to Wasabi CoinJoin for further anonymity, would the blockchain analysis companies working with Wasabi have the ability to detect them, or would BIP-352 actually be good enough to protect users' privacy?
No one knows. Wasabi will block whatever their government buddies tell them to block, and they won't give any reason for doing so. No point in risking it when you can just use JoinMarket or Whirlpool instead.