Hi. I would ask your accountant for a copy of their bank reconciliation at 31st March 2021. That could show up outstanding transactions at 31/03/21 (entries posted in March that didn't clear until April) and help with your first reconciliation at that date.
If they don't have one (!) then the only solution would be to post a correcting Journal as you suggest, on 1st April, but again, I would ask your accountant where to post the opposite side. You can then do one reconciliation up to that date to tick everything as cleared.