It's unclear from your post whether you've signed out of your original Apple ID or switched the email address of the account. However based on some words you've used I'm guessing you've changed the email address of one account.

If you set your Apple ID to a non-Apple email address, then you don't lose your original Apple ID email address. As you noted, it becomes an alias to the account when you go to sign in.

None of this prevents you from paying for subscriptions or accessing your data. You can continue to use the account. If you don't want the account attached to whatever email address you entered, then for the time being you can set up a new Gmail account and change it to that.

You should be able to remove the temporary email address you set as your Apple ID, but there's a possibility it's being restricted in this scenario. You'll have to contact Apple Support to confirm you're eligible to do that.

It's unclear from your post whether you've signed out of your original Apple ID or switched the email address of the account. However based on some words you've used I'm guessing you've changed the email address of one account.

If you set your Apple ID to a non-Apple email address, then you don't lose your original Apple ID email address. As you noted, it becomes an alias to the account when you go to sign in.

None of this prevents you from paying for subscriptions or accessing your data. You can continue to use the account. If you don't want the account attached to whatever email address you entered, then for the time being you can set up a new Gmail account and change it to that.

You may be able to remove the temporary email address you set as your Apple ID, but there's a possibility it's being restricted in this scenario. You'll have to contact Apple Support to confirm you're eligible to do that.