Toby
A conundrum we face daily. Working through the long lists detailed above is arduous work and I will suggest a start.
If you have access do a site audit with moz or semrush. If access to moz review the moz trust score. After a site Audit you should have a better understanding of some technical issues but there are two factors that I address asap page speed. Just checked your on tools.pingdom and it was below 2 seconds which is excellent.
The second aspect which I suspect has high relevance is bounce rate. I would imagine bounce rate could be a factor for your site from a quick look at the home page. I am not sure whether the site is a shop, info brochure on ear plugs etc. I did not see a CTA. So if your bounce rate is above say 50% I would be studying GA for behavior on the site. Work out what page customers are dropping off, and then dedicate your self to getting the customer to move to the next page. Follow them all the way through to a conversion. If not enough traffic ask family or friends - though they can be brutal.
There are plenty of other items on your list - but understanding your customer and how they are interacting with your site is vital - and creating a deep connection with the right customer is integral to sustained success. If people have a good on site experience they will talk about your site and come back.
Ask away for clarity on any point.