Cast local key in morphMany relationship before query

I am having a problem with a query in MySQL 8 as it's not using the index as it should. I have two tables, one is users and the other is conversions.

users (id bigint unsigned)
conversions (id bigint unsigned, actable_type varchar(191), actable_id varchar(36), INDEX actable_id)

Then, in the User class, I have the relationship defined as

    public function conversions(): MorphMany
    {
        return $this->morphMany(Conversion::class, 'actable');
    }

Here's how it performs:

> DB::enableQueryLog();
> $user = User::find(1);
> $user->conversions()->get();
> DB::getQueryLog();
=  [
      "query" => "select * from `conversions` where `conversions`.`actable_type` = ? and `conversions`.`actable_id` = ? and `conversions`.`actable_id` is not null",
      "bindings" => [
        "App\Models\User",
        1,
      ],
      "time" => 448.01,
    ],

> $user->conversions()->explain()
= Illuminate\Support\Collection {#8356
    all: [
      {#8252
        +"id": 1,
        +"select_type": "SIMPLE",
        +"table": "conversions",
        +"partitions": null,
        +"type": "ALL",
        +"possible_keys": "conversions_actable_id_index",
        +"key": null,
        +"key_len": null,
        +"ref": null,
        +"rows": 616308,
        +"filtered": 5.0,
        +"Extra": "Using where",
      },
    ],
  }

When I run the query manually where I pass the user id as a string, it performs like this:

> DB::enableQueryLog();
> Conversion::query()->where('actable_type', 'App\Models\User')->where('actable_id', '1')->get()
=     [
      "query" => "select * from `conversions` where `actable_type` = ? and `actable_id` = ?",
      "bindings" => [
        "App\Models\User",
        "1",
      ],
      "time" => 7.93,
    ],

> Conversion::query()->where('actable_type', 'App\Models\User')->where('actable_id', '1')->explain()
= Illuminate\Support\Collection {#8356
    all: [
      {#8381
        +"id": 1,
        +"select_type": "SIMPLE",
        +"table": "conversions",
        +"partitions": null,
        +"type": "ref",
        +"possible_keys": "conversions_actable_id_index",
        +"key": "conversions_actable_id_index",
        +"key_len": "145",
        +"ref": "const",
        +"rows": 1,
        +"filtered": 10.0,
        +"Extra": "Using where",
      },
    ],
  }

As you can see, when the actable_id is passed as an integer, it doesn't use the index, but when it's passed as a string, it uses.

I need the actable_id column to be a varchar(36) because sometimes the actables have UUIDs instead of bigints. How can I instruct Laravel, ideally in the User model class, to cast the User key to a string? I only want to do it for this relationship so it doesn't affect other relationships.

I can do the queries manually all the time, but having the relationship doing the right thing would simplify the code a lot.