For 10,000 PLN you currently get the most USDT on Revolut X: 2,560.17 USDT. The least favourable exchange (Coinbase) gives you the equivalent of 47.24 PLN (0.47%) less.
Exchanges with a direct USDT/PLN market: Kanga Global. Elsewhere you need a currency conversion or an intermediate currency, which usually adds cost.
Counting only the trading fee and slippage, Bitstamp wins, but once deposit, conversion and withdrawal are included, Revolut X comes out cheapest.
For 1,000 PLN the best is Binance, for 100,000 PLN it is Bitstamp. With large amounts order book depth matters more than the headline rate.
Even for 100,000 PLN, splitting the order across exchanges brings no benefit, so one exchange is enough.
PLN deposits: P2P (Kanga Global); BLIK (Bybit EU, OKX).