It's impossible for us to say with no access to your account, but from the points eBay have raised and you confirm no returns, and all tracking updated, it must then be one of the other reasons.
Perhaps an old fee has not been made, perhaps some further documentation is required. Check your email and eBay messages and spam, as others are reporting recently receiving notification to provide NI numbers.
Only Customer Services can tell you . I know you have asked them before, but contact CS again and only very first thing in the morning when the lines first open preferably on a weekday at 8 a.m. as there's more chance of Dublin answering, only speak to them. Ask what exactly is required from you, for you to be able to use your selling account fully again.
This is the easiest and quickest way to contact eBay Customer Services, for a Call Back option.
https://www.ebay.co.uk/help/eua?id=5275&mkevt=1&mkpid
Lines open between 8 a.m. to 10 p.m. on weekdays.
9 a.m. to 6 p.m. on weekends.
I recommend contacting CS very first thing in the morning, when the lines first open, as there's more chance of Dublin answering.
@theeternitiveltd