Moneydance has been working on Big Sur for a while. It is best to install the latest preview release and at this stage to disable tabs in the Mac System preferences, hopefully this restriction will be removed at some point.
Quickbooks is a problem, Intuit does not really want you to move away so they do not provide the tools that readily allow it.
QuickBooks doesn't allow exporting to QIF, which is the main format that is used to migrate between software programs. Only the Intuit Interchange Format (IIF) is supported.
An IIF to QIF converter will not solve the problem either, as journal entries can't be exported in IIF format, only lists are exportable.
A somewhat viable way to overcome this problem is to set up a journal report, to show all journal entries. Print the report using the "print to file" option. Set the file type to Excel before printing. Rename the extension of the resulting file from PRN to CSV.
The CVS importer in Moneydance may be able to handle this file, I do not know I have never used Quickbooks and have seen no references about these files, or a sample from Quickbooks.
You may be able to use the XL2QIF Excel macro to convert to QIF. The Excel file may need to be reorganized to generate the appropriate format for the macro to work, such as separating cheque accounts from term deposits, etc. The above referenced Excel macro supports split transactions.
All in all it would require some work to move the data from Quickbooks.