We do have a trial balance report in the Reports section of QuickBooks Online that shows the overview of the accounts
However the best way to import mid-year balances would be to use our Chart of Accounts import feature to import your accounts from Wave, and then use the Journal Entry import feature to bring over the balances. Both can be accessed from the Gear Icon, then Import Data under the third column.
From those pages, download our sample .CSV file to map the fields from Wave and into the .CSV file, then upload it through the Import Accounts section.
If you're entering the opening balance in the Chart of Accounts sample .CSV import file, just enter the figures as positive or negative from your Wave balances, and what that will do is create an opening balance transaction for that figure. Note that importing opening balances via the Chart of Accounts import is only applicable for Balance Sheet accounts.
For P&L accounts (expense and income accounts), first ensure you've imported the account names in with the Chart of Accounts import, and then you will need to use the journal entry import feature to bring their opening balances in.