Thank you for the comprehensive response.

I had never heard of a 303 redirect until I discovered today that I had them all over my site almost by accident, so I've learned a major lesson learned on getting input from an SEO specialist before undertaking any major website work with my developers because clearly there's a lot I don't know and they don't know... worse, I don't know what I don't know until something goes wrong!

I didn't expect I'd be able to get any of my link equity back after a year of trying to find the cause of the problem, but I will definitely have the redirects changed ASAP just in case there's anything left of it!

Thanks again for the helpful advice.