Why did only the last two months of orders come across?
Because Shopify limits order access to the last 60 days unless the app has been granted the wider order permission, which is a separate scope you request on the custom app. The import reports this in its warnings rather than leaving the short history looking like a fault. Grant the scope in Shopify and import again: the older orders join the same table, beside the ones already there.
Should I be replacing Shopify with this?
No, and nothing here would let you. Shopify runs the storefront, takes the money and handles checkout, all of which it does far better than a workspace builder would. What it has no opinion about is what happens between the order landing and the parcel leaving, which for a lot of merchants is arguably where the actual business is.
Does it keep up with new orders automatically?
Yes, on a schedule. Importing again adds new orders to the same table and updates changed ones, a fulfilment or a refund for instance, matched on the Shopify order ID, and Keep in sync runs that hourly or daily. Without the wider order permission each run sees only the last 60 days, so an older order stays as it was when it last fell inside that window. If you need to react within seconds of checkout, that is a webhook problem and a different mechanism from an import, so it is worth being clear before building a process that assumes it.
What does the stock step actually change?
The available quantity of one inventory item at one location, by an amount you give it. It does not set a total and it is not a product-level operation, which surprises people the first time: the inventory item id is a different number from the product and variant ids, and in a multi-location shop the location id decides which warehouse moved.
Does this work right now?
Yes. Shopify connects with credentials from a custom app you create in your own admin, so nothing depends on an application we registered with them and there is nothing pending at our end. Create the app, grant the read scopes you want, paste the values, and the resource list appears.