QB Issue Resolution:

as it seems like you can only do sub-accounts under the same account type

Correct accounts and sub-accounts must be the same type, there is no automatic assignment of sub-accounts..

I cannot find a general category for Assets, Expenses, or Liabilities on a top level–is this assumed and not assigned based on the available account types?

This is setup based on the business type selected when you started the company file. You can check the sample company files and see how they are set up.

Resolution for Issue 'Top Level Chart of Accounts Not Available When Setting Up New Accounts' available: Yes (Solved).
Source: Intuit Community forum.

Top Level Chart of Accounts Not Available When Setting Up New Accounts: this issue or error code is a known issue in Quickbooks Online (QBO) and/or Quickbooks. Support for this issue is available either by self-service or paid support options. Experts are available to resolve your Quickbooks issue to ensure minimal downtime and continue running your business. First try to resolve the issue yourself by looking for a resolution described below. If it is a complex issue or you are unable to solve the issue, you may contact us by clicking here or by using other support options.

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