You have several options:

1 - IDEAL / NO WORK AT ALL

It may be possible to keep a single read only license to your old system

• Pros - easy to look things up, nothing new to learn
• Cons - it is another system, so now things are in two places

2 - QUICKEST / EASIEST

Import Excel sheets of the relevant information into Odoo Spreadsheets via the Documents App

• Pros - get access to what you need without much work to import
• Cons - harder to search for things, not integrated well

3 - MORE WORK

Create a Studio App and import tables of CSV data

• Pros - can use Odoo filtering and search, form and list and kanban views, can link to Customers via Smart buttons
• Cons - takes more time

4 - EVEN MORE WORK

Import Monthly Trial Balances

• Pros - you can run year on year comparisons
• Cons - takes even more time

5 - EVEN MORE WORK

Import Transactions that you later cancel

• Pros - Invoices without Journal Items.
• Cons - VERY VERY VERY time consuming, needs scripts, very advanced knowledge required, may not have a good return on investment 

Needs the Invoicing Switch Threshold date (Accounting Settings)

6 - THE MOST WORK

Import Transactions

• Pros - works as if you were using Odoo the whole time
• Cons - VERY VERY VERY time consuming, needs scripts, very advanced knowledge required, may not have a good return on investment 

Resources for going live with Odoo Accounting:

https://drive.google.com/file/d/1QJ_nRLNq1kjqX-oapYNo_3JYBpnFgap2