The TCA is a basic trade deal that eliminated tariffs on goods that meet the relevant rules of origin but did little to reduce non-tariff barriers. If Scotland were to join the EU it would trade with England and Wales on the same terms as other EU member states do now, so new checks and processes would be required on trade across the Anglo–Scottish border. It would also mean that border infrastructure would need to be erected on the Anglo–Scottish border, across which there are 21 road and railway crossings.32This is a short paper that caught my eye the other day, published by the Institute for Government and available for free here. I'm neutral tending to positive on Scottish independence, and felt that this paper laid out well some of the issues regarding Scottish frontier policy that would arise inevitably from an independence status, especially in the light of Brexit (which was not an issue during the 2014 referendum). Basically, it's impossible to see how an independent Scotland could avoid frontier controls with England (and therefore Wales), whether or not it joins the EU, because of the particularly hard form of Brexit that Boris Johnson chose, unless it chose to remain essentially a vassal state of London (which would also require London's collusion). The desired end-state of course is EU membership; the authors are I think a little pessimistic about how long that would take (though they do slay the myth of the Spanish veto), but clear that this would certainly mean that the trade frontier between Scotland and England would end up looking much the same as that between France and England, or Ireland and Wales. Northern Ireland is of course a different matter, but the authors rightly do not devote too much time to that as it's not a Scottish issue. I think it's also worth pointing out that the 21 border crossings between Scotland and England would be pretty easy to police, and the landscape is favourable, unlike the situation in Ireland.
32 HM Government, Scotland analysis: Borders and citizenship, Cm 8726, The Stationery Office, 2014
The authors also look at other alternatives to EU membership that Scotland could try, but one comes away with the sense that there is no real middle way; Scotland can choose continued Union with England, Wales and (to an extent) Northern Ireland, or independence which inevitably means economic disruption to its relationships with the rest of the UK. A renewed relationship with the EU will partially but not completely substitute for that, and there's not much point in considering anything other than EU membership as an end point. Scottish Nationalists should not pretend that independence will come without a price; of course, the lesson from other cases (including Brexit) is that voters can be persuaded that it is a price worth paying.