Wordpress site incompatible with theme update

I made a mistake by trusting a Wordpress maintenance company run our website’s updates since 2021. Including theme updates. I sent them the 2025 theme update files and they responded saying it was completed. A week later, I realized I couldn’t edit theme settings at all. I reach out to them and find out that the theme update was actually never completed. They said it is incompatible bc it has been too long since the last update. Turns out, they never updated it over the years. They recommended their “partner” to rebuild our website. Never offered to try a previous update or offered any other solution. Our website has been suffering in SERPs and struggling performance wise - I couldn’t figure out why. But I am attributing this to an outdated theme. Even with WP rocket installed, our site has a mobile lighthouse score of 68. I need advice on what to do from an SEO perspective. I have renewed support for our envato theme and contacted the developer - waiting for a response. My options are: - rebuild website - have developer patch the issue - continue using a my theme as a legacy theme Our rankings have dropped about 5 spots across the board for many of our keywords with last algorithm update. My concern with rebuilding/switching themes is that this will negatively impact seo rankings. I have built up strong backlinks over the years and just landed some more high DA backlinks but I fear that this will not be enough to offset the technical seo issues.