There doesn’t currently seem to be any way to get a 90hz/30fps cap via Gamescope without using the unified slider because the only cap options for 90hz are 90/45/22.5. You can get 90hz/30fps with the unified slider just fine, but if you aren’t using it then you have to drop down to 60hz for 30fps to be an option, which introduces extra input lag. Ideally it’d be nice to just have a free FPS limit slider when the unified slider disabled since the ease of use aspect of it is handled by the other mode now.
Try that : https://github.com/ryanrudolfoba/SteamDeck-RefreshRateUnlocker