Given how important search rankings are to your decision, I'd pay close attention to the SEO migration itself. Switching website providers doesn't necessarily mean starting from zero, but a poorly handled migration can cost you rankings you've already built.
I don't know much about the specific providers you're considering, but I'd make sure whichever provider you choose has a clear migration plan for preserving your existing URLs, redirecting old pages where necessary, and maintaining your existing SEO content and metadata.
When you say you've lost rankings, are you referring to specific search terms, individual vehicle listings, or a general decline in organic traffic? Did this happen gradually with your current provider, or did it start after a website redesign or migration?