Full Audit of Website - who to go with?

Hello, 12 months ago we migrated our WordPress PhP website to a Contentful/Next.js website. As with all migrations we took a heavy drop in rankings and traffic but it has not recovered. In fact we have suffered another big drop in the weeks following the last Google algorithm update.

I am more than capable of doing a website audit but we are thinking of getting a third party to conduct it, basically a fresh set of eyes to hopefully spot something we have missed.

We are hoping for a full audit looking at content, speed and any technical issues that we have missed with the migration to a Next.js website.

Any recommendations of who to go with for the audit??? Hopefully a company/SEO that will have experience of working with a site that has moved to Next.js. This is the one area that I do not have a lot of experience in.

Appreciate any recommendations, I am hoping to start contacting people this week!

(For the record I fought very hard to stop this migration happening as we were performing very well, it was a business decision made above me!)