Thanks for this useful info. I've done some more digging however, I may have just stumbled across what could be the issue in the slow paced decline month on month...?
So back when we started to gradually loose SEO ground we were actually changing URL structure from
fishingtackleshop.com.au/categories/fishing-tackle to fishingtackleshop.com.au/fishing-tackle (we removed the /categories part of the URL so link juice wasn't being passed onto that benign sub-directory "categories").
However, in a Screeming Frog Crawl today what i noticed but haven't picked up on before since i was only looking for 404 and 301 issues, is it seems we are actually having canonical issues.
SO,
/fishing-tackle is not indexed in google since it is canonicalised to /fishing-tackle/ (trailing slash).. Why i don't know perhaps as developer has listed trailing slash link in the menu.
but /fishing-tackle/ is also not indexed when i just did a google search.
So, I am guessing i may have found my issue? (or a big part of it)!