I set a start date for a bit before the beginning of this fiscal year and have entered all relevant data back to that point.
I do, of course, have a lot of data from previous years available. In the back of my mind, I’ve wondered “What would happen if I set the start date earlier on these open books and entered those transactions?” (assume for purposes of this question that I properly adjust the opening balances in each account). Would that work ok, or would it break something fundamental? I’m particularly curious about the impact of this on any bank reconciliations I’ve done (I haven’t actually reconciled at all yet, precisely because I want to understand this issue first.)
There is no problem in entering data from previous years. You need to enter starting balances for the beginning of the period you want to enter, then enter your previous transactions.
Unless you have very few entries, the effort involved may not be worth it but only you can decide that.
In particular, there will be no effect on bank reconciliations. Think of that question this way: if your cleared statement balance in Manager matches your bank statement on any given date, you are reconciled as of that date. Pending transactions, even if entered incorrectly or incompletely, have no influence on that because your bank does not know about them yet. In reality, that also means there is no benefit to creating any bank reconciliations for past periods.
Manager is a continuous accounting system which I believe mean calculated data is dynamic based on all prior entries not just for the current financial year.
As such I’m not aware of any reason you could not
change the start date to an earlier year
change the starting balance to match those of the earlier year
import transactions for the earlier year
check reconciliation for the earlier years
After which the later period should also reconcile.
Of course you do this in a backup business just in case you decide on a different approach halfway through.