You won’t find much better opponents on old duel servers vs Official Duel servers. The ranks are higher on Old but for the most part the standard (these days) is the same. In fact I would recommend you to stay on official Duel mode if you are playing on Europe because the classic duel servers in EU is a messy experience of FFAing - Players rank 40+ (thats about 500-600 hours game) Still debating feints in chat, vote kicking and players refusing duels.
The official duel mode is a overall better experience and for the long term enjoyment I recommend the official.
This. I just tried a old duel server (TG | OLD DUEL something) and the admin (Coral) kicked me after I won a duel against him. He said “wow much skill” then just kicked me. I wish I knew if he was part of some clan I could report abuse.