return specific columns

The User instance is already fetched, so query methods (select and get) are useful only if you query for the authenticated user again.

Route::middleware('auth:sanctum')->get('/user', function (Request $request) {
    return User::select(/* columns */)->find($request->user()->id);
});

Otherwise, you can return an array representation of the User:

Route::middleware('auth:sanctum')->get('/user', function (Request $request) {
    return [
        'id' => $request->user()->id,
        'name' => $request->user()->name,
        'email' => $request->user()->email,
        // ...
    ];
});

@sguserface You can use makeHidden method to hide columns.

Route::middleware('auth:sanctum')->get('/user', function (Request $request) {
    return $request->user()->makeHidden([
        'id', 'name', 'etc'
    ]);
});

https://laravel.com/api/8.x/Illuminate/Database/Eloquent/Concerns/HidesAttributes.html#method_makeHidden