1. Run a search on all transactions posted from 1st April to present
2. Change all transaction posting periods to Mar 21
3. Delete FY 22
4. Extend FY21 to 31/8/21
5. Move all transactions back to respective posting periods
6. Create a new financial year from Sept 21- Aug 22.

You cannot set a default custom report, it will always default to the standard reports.

I would advise prefixing it with the company name and then adding it to the menu.
Alternatively you can force it through on Shortcuts or Dashboard across the company.