Exfil all your data and find a new service. Not hard. Cost of business with any start up, be prepared to exfil in 3 mo at any time. Run into this over and over.
Many (most) packages have an API, but transferring data in and out requires deep knowledge of systems, their special pricing rules, their tax rounding rules (which is also different for every country) and many many other things, it was difficult enough for someone with years of experience in this domain to get right, the chances of even a senior developer doing this properly without field experience is zero.
(It's a nontrivial problem but there's at least a reasonable check—"Did the balance sheet, P&L, and cashflow statement generated by both systems match")