There won't be 7,000 trucks in a queue to Kent
In order for a UK haulier to take goods to an EU member state they need an ECMT permit, these come in 2 flavours, monthly & yearly.
In March of 2019 the UK had 11,392 applications for yearly permits of which it allocated 984 with a possible 70% more ( 689 after rounding up ) to come so that's a maximum of 1673 annual permits.
They also had 2,832 monthly permits which hadn't been allocated.
So in any one month there's between 3,816 & 4,505 permits, which translates to the same number of UK vehicles travelling to the EU, no point setting off for the EU without a permit.
I doubt 2020 has a much larger allocation, given the state of talks.
And 11,392 applications were made, and that's when we had FoM and didn't actually need permits, we just thought FoM could end any day so we'd best get them, when transition ends those permits will be needed.
And that's for the entire UK, any port, not just the ones in Kent, we'll sharing those 3,816 to 4,505 trucks between several other ports as well as Dover.
So if there's 2000 UK trucks going through Dover we'll be lucky, though that figure's just a guess. I'd be surprised if UK haulage operators weren't looking at alternative routes to avoid queueing though. Any RORO ports that happen to be on the east of the UK & aren't in Kent will do see https://www.gov.uk/guidance/list-of-roll-on-roll-off-ports.
And any EU trucks that get stuck in a queue returning to base will only do so once as their operators won't send them again, sitting in a queue costs them money, UK haulage will be uneconomic so they'll finish their current contracts & won't pick up any future ones, after all they have 27 other countries they can haul to.
The Government will be well aware of all this as they're the ones that issue the certificates in the UK. I'd say they're just going to pretend they don't know and act all surprised in January in order to whip up some anti EU sentiment, "But we built all these car parks, disrupted lives, put a border round Kent & now the nasty EU won't even let us use them waaaaah, it's all their fault how could we have foreseen this" yadda yadda
Source for March 2019 figures - https://www.bbc.co.uk/news/business-47377037