As far as I am aware, there hasn't been an exact consensus on the Nuzlocke rules for Sword and Shield. Even Bulbapedia hasn't been updated to reflect the differences in these games compared to the others.
I did find this guide that can shine some light on the major differences you mentioned. A lot of the same rules still apply, and the common ground from the other games is that it is up to the players discretion on how they want to proceed.
Pokemons are not random encounters in tall grass anymore. They just walk around and you could select one before running into it.
In that zone you can find pokemon that are way over your level and cannot be caught.
Since there are no longer random encounters, you choose which Pokemon you want to capture. Either it be one that is hidden and rustling in the grass or visible and wondering around, it is your chose on which one you want to capture.
Just be sure to avoid the "Very Strong" encounters early on, as those require additional gym badges to capture. If you follow the Nuzlocke rules to the letter, then this would be your "first" encounter and therefore lock you into catching an impossible capture.
The guide I linked mentioned that you have the option to use a raid dungeon as your first encounter if you choose to do so. But if your party wipes and the raid boss flees, then you lost your first encounter in the that zone.
A lot of roads have been replaced by a huge, open, zone
While the wild area is one giant area massive, the answer to this one is quite simple. The wild area is split into different zones with different biomes. Each zone will follow the catching rules according I mentioned above. Routes and dungeons will work the same as well.