Haven't used banking function for years and now want to reconcile and clean up my account

I'm in the UK and have been using Zoho Books for my private mental health practice since 2018. Up until recently, I've entered everything manually and not reconciled any items with my bank account. Every year, I run a report for that year and use that for taxes.

I didn't enter starting capital early on, but just moved money from my personal account to pay for things as needed. I have records of everything I paid for, but haven't accounted for where money came from to get things up and running.

Zoho Books now shows a huge cash balance and I want to reconcile it so that it reflects my actual bank account, which is nowhere near that sum. I'd then like to move forward with my bank connected and just match transactions every month.

I've added my bank account, but the feed only goes back three months. My bank has records all the way back to 2019. I don't think I have my previous bank records from 2017-2018.

I know almost nothing about accounting systems, though I've always kept meticulous records of all incoming and outgoing so that I could do my taxes properly. Is there a way to start over again from this tax year, or, even better, a quick way to reconcile all previous transactions than to enter them manually?

I'd welcome any advice on how to approach this since it feels very messy. Ideally, I'd like to clean up my accounts all the way back, including entering start capital and my transfers to myself (I'm a sole trader). I'm also anticipating HMRC's new system starting next year and would like to have everything in order because of that.

Thanks,

Peter