10-year old site with a 20+GB library – want to decrease size

We have been running our site for 10 years now and we have accumulated a lot of content in that time. Over the years, we’ve used different themes, plugins, etc. My partner and I are not developers and our knowledge only goes so far. We have a very larger library, but we aren’t sure what is actually necessary for our site and what is bloat from changes made to the site over the years. We don’t want to just look for a person on a site like Fiverr (there’s nothing wrong with it – we just want to make sure that we have a good resource and not a random one). Does anyone have any suggestions or resources that would be good for this sort of thing? If anyone happens to have knowledge of what we should look for in our library – files we wouldn’t need anymore – that would be much appreciated.