Setting up opening balances for income and expense accounts over-complicates things and Manager doesn’t support this (and never will).
If you want to start on Manager in the middle of the financial year, then for that year, you won’t be able to produce P&L for the whole year. You will need to take P&L from previous accounting system (e.g. first 6 months) and then P&L from Manager (e.g. last 6 months) and combine those figures yourself.
Then next year since it will be full on Manager, you can get P&L for the whole year.
Other option is to ignore whatever was done in the previous accounting system this financial year and set Manager to start at the beginning of this financial year and re-enter your records in Manager. This is actually what most people prefer to do anyway so they have full year in Manager even if they start using Manager 3 months into new financial year.