Is becoming a Shopify Plus Partner essential for app development?

Please forgive me about my unsophisticated question since I never used Shopify before.

We are an application vendor and trying to develop an EC site for a client using shopify Plus, but we’ve never used it before so I heard that we have to be Shopify Plus Partner first, and so now have questions below.

1、I heard that we can’t access to the Shopify Plus sandbox store unless we are the partner, but is the creation of a sandbox store essential to the development process? (Is there any other development environment besides sandbox?)
2、If a vendor has a high level of expertise and doesn’t need support from Shopify for development support or advanced customization, and if we don’t need to edit liquid or implement our own apps,
a vendor doesn’t need to be a Shopify Plus partner?
3. Isn’t the Shopify Plus account manager assigned to the client side complementing 1 and 2?

Any update on this? I am app developer and trying to use some Shopify plus features, how can I do that?