Like I said, with the right technology in place I'd reconsider. I'm not emotionally attached to the union as many seem to be and the rest of the UK gets a terrible deal from Scotland in so many ways. My primary concern is that upon becoming independent Scotland might suffer a severe decline in living standards that would result in a large and sudden influx across the border, putting huge strain on England. A hard border would potentially solve that but at very high cost and significant inconvenience: one based on a solid ID system would be much better.
I don't recall any talk of ID cards or transitional periods to sort things out during the Scottish Indyref. Perhaps I missed it.
The reply to the open letter is what you'd expect and isn't reassuring. It makes a few errors.
Firstly, it's not looking forward. It states the EU today is not as nearly as bad as the USSR was at its worst. That's true and nobody argues that it is. However it's also not really relevant, because if you only start to object to a government at the point it's running gulags it's already far too late. To avoid a repeat of history, movement towards such a system must be detected and stopped early whilst it's still possible. That's why we need to look at the direction of travel and extrapolate.
Secondly, it's looking backward. It talks about how the UK was able to opt-out of the EU's worst ideas and talks about Thatcher. That was true in the past, but the EU has been getting more powerful over time and taking over more and more national responsibilities. The veto has been progressively weakened and now they want to abolish it entirely. Today's EU is not the same as the EU in Thatcher's day - it's significantly larger and much more hostile to "deserters", as Juncker has described the UK. And as the EU loves to keep telling the UK there is no more "cherry picking" anymore, i.e. opting out of their bad ideas. That era is over.
So what matters is what the EU will become, not what it is today or what it was 20 years ago.
And here is where the comparisons start to look bad.
Firstly and most importantly the EU does not recognise the democratic will of the people, whilst loudly proclaiming that it does. This is a major red flag. There is no effective way to control the Commission's agenda through voting.
It must be noted here that all communist regimes have Parliaments and they all have votes. In the USSR there was an elected body called the Supreme Soviet, whose members made the laws of the Union. The USSR was a dictatorship despite the existence of the Supreme Soviet because of a simple trick: all elected politicians had to belong to the same party, so there was no real competition of policies. It had the superficial appearance a democratic system without being one.
In the EU there are multiple parties and they do compete. The EU Parliament is neutered via a different means: the elected politicians can't control the executive or change the laws. This means they aren't really politicians and it's not really a Parliament, using standard dictionary definitions. It wouldn't matter if somehow a coalition of parties totally opposed to the Commission's agenda got elected: the Parliament lacks the "right of initiation". That also means there's no real point in having any party policies, so you see a wafer thin and artificial campaigns, where parties either have detailed manifestos that happen to align exactly with what the head of the Commission already wants, or you get protest parties that openly admit they can't change anything but say: elect us anyway just to 'send a message' (which is never received).
A good example of how unseriously the Parliament is taken is the way they were given a "vote" on the new head of the EU Commission, but the vote only had one option. MEPs could either support Ursula von der Leyen, or abstain. Votes with only one option are a classic symptom of communism; the sort of system that is happy to call itself a "People's Republic" but actually doesn't trust the people.
By the way, the EU does try to obfuscate a lot of this stuff by introducing mechanisms that theoretically are usable but in practice aren't. For instance there was briefly the Spitzenkandidaten system which meant the Parliament appointed the head of the Commission ... up until they appointed the wrong one, at which point that ability vanished. There's theoretically an EU Citizen's Initiative, but unlike Citizen's Initiatives in member states, this one can't be used to change the law or trigger a referendum. Actually they accomplish nothing even if you gather the required number of signatures (which is huge).
In the end the true nature is summed up by Jean-Claude Juncker: "There can be no democratic choice against the European treaties".
Another bad sign is the love of secrecy. In theory your countries national leader can influence the choice of Commission leader, except the decision of who gets the job is always made in secret and nobody knows who voted for whom or how the decision was made. This matters: the EU's own leaders have said in the past it's common for politicians to lie to their own voters about what they support. There is no public explanation how Ursula von der Leyen became head of the Commission. It could be fixed by making EU meetings be transparent but they refuse to do it. This pattern of secret meetings sans minutes crops up everywhere when you scratch the surface.
There are lots of other pieces of evidence the EU doesn't care about votes or democracy. For instance, in a democratic system you might have expected rejection by one of the biggest members to trigger at minimum some sort of reflection or debate. There hasn't been any - at the first meeting of the EU Council after the Brexit vote it wasn't discussed at all. The only topic on the agenda was creating an EU army - one that would report directly to the Commission.
Why does the EU need an army when there is already NATO? It doesn't, but totalitarian regimes do. Expect to hear more about this army's "peacekeeping" initiatives in future, albeit not for many years. The process of transforming into a Neo USSR is a slow one, fortunately.
The EU appears to implement rule of law, until the law gets in the way of the EU's aims and then it simply gets ignored. To quote Christine LaGarde on the Eurozone bailouts: "To save the Euro we broke all the rules. The treaties are quite clear. No bailouts."
Finally, loyalty to the EU strongly resembles the sort of loyalty to communism seen in the first half of the 20th century. Brexit has made this very clear by flushing it to the surface. Strongholds in academia, a belief in the superiority of 'expert' run government over democratic decisions, constant double-speak like a "People's Vote" (of the sort held in a People's Republic), a willingness to do whatever it takes to gain victory. Orwell had a lot to say on these topics.
I could go on but you get the picture. The EU looks superficially democratic and law-based, but blocks actual democracy at every turn and the law is enforced politically. It's a deep seated set of attitudes that can't be reformed. It will inexorably continue on its present path, with the EU getting steadily more powerful until it totally controls the member states, helped by large numbers of quasi-religious loyalists who will crack down and imprison anti-EU protesters under the guise of "hate speech".