As you pointed out, there will be a sizeable bottleneck at the border, and this will be no small matter, as delays may well apply to both persons coming from the EU, and vehicles. We've already seen the effects of problems at Dover port and the channel tunnel, and the somewhat limited response in the UK known as 'operation stack'.
More border officers are part of the solutions, but time is very limited now and even with 6 months - 1 year there are only so many border officers that can be hired, even if there was no point of diminishing returns in terms of how much faster more staff can operate the facilities and the border.
In addition to this, there is significant concern from businesses about how they will be able to get product and materials in and out of the country in a timely fashion; this has included concerns about critical goods such as medicine.
Having more ferries and more ferry routes to different ports is unlikely to solve the above issues completely, but will help alleviate them. It will also give the government additional options such as using the additional capacity for priority shipments.