This is Elias with Shopify Support. Thank you for reaching out.
Since Simple is a Shopify-supported theme, I can confirm collapsing the dropdown items in the side menu by default is possible and our Theme Support can help you with that.
If your store is subscribed to a plan, you’re eligible to use the complimentary design time for customizing your theme. This service will be free of charge with your subscription and I’d be happy to help send a customization request to our Theme Support team on your behalf. It’s important to note, there are some customizations that we’re unable to support and we’ve outlined them on our design policy.
Can you confirm if you’ve subscribed to a plan? If not, did you get a chance to review our plans and features?