Do they? Since Independent Scotland is very likely to rejoin EU it seems to me Russia & co would be interested in keeping it on the sinking ship that is post-brexit UK(economy wise).
And while I certainly think it's fair to describe the UK economy as a sinking ship, I also think that blaming that on Brexit is, to put it politely, "starting with your conclusion". UK growth has been higher than France, Germany or Italy since 2016. Brexit has obviously had impacts, but they haven't all been negative (the City in particular has zero enthusiasm to fall back into any EU alignment) and I think the COVID lockdown shambles and the Homerically inept current government have been bigger factors.
I found this a decent recent overview on the common analytical takes, if you're interested: https://julianhjessop.substack.com/p/what-the-nber-gets-wron...
Covid has been global, lockdowns have been everywhere, UK is not unique and did not even have the worst of it in terms of lockdown strictness. While "averaged out" UK economy post-brexit/pre-covid might not look that much worse than EU, if you look into specifics the picture gets far uglier with entire economy sectors going bankrupt, all in all it was a spectacular self inflicted damage that will be felt for decades to come, especially now that US is becoming a hostile actor.
The Scottish independence referendum was in 2014, two years before the Brexit referendum.
I have lived in UK during the referendum, I remember it vividly, nobody seriously believed that it would actually go through, it seemed THAT absurd.
I am still convinced that brexit is one of the first big wins of Russian meddling campaigns.
Though its notional deficit would be far higher and break EU rules IIRC (which have been broken numerous times by existing EU states).
That is in no way clear because Spain has previously (during the Scottish independence referendum) indicated they would veto such a thing (because they don't want to give Catalonia any ideas or hopes).
But in any case, a Scottish independence would force the UK to find, and invest a lot in, a new base for its nuclear submarines. Scotland is also the most important UK part of GIUK gap, where there is a lot of infrastructure to catch and track Russian assets entering the North Atlantic. UK losing access to that is not good for it nor for the US with their special relationship.
A Scotland in the EU can also create tensions between the UK and the EU, which would also be good for Russia.
At least for several decades after independence
The level of integration of everything, including trade and supply chains and finance etc, is so big as to be almost total.
In addition, I'm pretty sure that EU would not welcome another hard border with England, it's already waisted far too much time and effort on the issue in Ireland. I'm sure it doesn't want to have to go through that again.
Scotland is of enormous strategic importance due to its location relative to Russia's naval ports. An independent Scotland with no other backing would have minimal resources to monitor and deter Russian naval activity.
I note that if you're looking for a weakly defended EU country reachable from the North Atlantic and quietly relying on the UK defence umbrella without admitting it, Ireland is already there.
I'm also not talking about the defence of Scotland itself, I'm noting that monitoring and curtailing Russian naval activity in the Baltic Sea corridor is of wider strategic importance. If Scotland became unable to do this, Russia would have an easy exit path for naval vessels from its Baltic Sea ports.
In your example, Ireland as an EU member has significantly more access to military resources than otherwise.
If anything it would force UK back into the EU, further strengthening it.