English nationalism and Scottish nationalism come from two very different places, and really aren't as comparable as you're implying. While it is obviously true that there are some Scottish people who do use it as a vehicle for ugly tribalism, this is far from the norm. Scottish nationalism is broadly a progressive project, despite the term implying otherwise.
When I lived in Scotland I met quite a lot of other English people who had shifted to a firmly pro-independence opinion. Clearly, given most would describe themselves as British or English, they didn't come to that opinion through any traditional sense of Scottish "nationalism".
I see this happen time and time again with people who are pro-union (and, for the record, I don't have a settled view one way or the other). English nationalism and Brexit are not as applicable to this issue as many think, and constantly looking at it through that prism misunderstands the pro-independence vote. It really isn't about flags for the vast majority.
Maybe, but that situation was specific to Brexit. The point is that Scotland would have every incentive to make negotiations as straightforward as possible, and there wouldn't be a powerful, hardline anti-EU block trying to sabotage that process in the Scottish Parliament.
You cannot call any strain of nationalism "progressive". Scottish nationalism may think its left-wing and liberal, but its nationalism is defined from the notion that it is left-wing and liberal
in comparison to England. Which means it still needs, like all nationalist sentiment, an "other" in order for its national identity to define itself. It's why, as an English/Welsh person, I view Scottish nationalism with deep mistrust and think the Southern left are extremely petty in embracing it as a reflex against Brexiteers. It's also why I don't sympathise with the idea that Scotland is "ruled" from England. It is not. It sends its MPs to what is effectively England's Parliament that have influence over lawmaking that only affects England. Scotland holds the key to power for the victory of the Labour Party. If Scottish nationalists complain about Tory governments imposed on Scotland brought into power by England and Wales, then campaigning for PR is a perfectly reasonable solution to that problem rather than all-out independence. The Scottish nationalist promise that after independence, Scotland and England would be two close chums is complete rubbish for anyone that spends more than five seconds thinking about it. England will control Scotland's main import/export corridors, that alone is a huge flashpoint for tension in Anglo-Scottish relations which could easily turn into a "England is trying to get revenge over Scotland for independence" argument.
In regards to your second comment, I think you're looking at the situation too optimistically. Negotiations between London and Edinburgh after a "yes" vote will be something we need as much as a second sun in the sky. They will be fraught, long and difficult. Aside from the fact I believe a Yes vote would be won on a less than credible nationalist agenda, Scotland would find itself arguing against a UK who has absolutely no interest in being lenient and showing "goodwill" and will, just like the EU, probably play tough. Whilst the nationalist side would likely splinter into more radical factions who will demand Scotland just "walks away" like the hardline Tory Brexiteers and moderate nationalists who will continue to negotiate. I doubt a Yes vote would be won with more than a few percentage points north of 50, so Scottish Unionists would just dig themselves into a fight which the Scottish Tories would have nothing to lose from fighting. It would be an absolute disaster for the people and security of this island and it brings me pain that people like Nicola Sturgeon would rather go down that road rather than use her unique position as a powerful public figure, to strong-arm the UK government into meaningful reform which we would all enjoy across the UK. The reason, of course, is plain old nationalism - its ultimately an "us and them" mindset, not a pragmatic mindset. And no amount of social democracy is going to wash that off the SNP.
Finally, I'm pretty sick of the hysteria around English nationalism. Yes, there is a sinister strain of English/British nationalism, but the broad, sweeping complaints of "English nationalists that voted for Brexit" and "white van men with 'Eng-ur-land' shaved into the backs of their head" smacks of classism and regionalist discrimination. I.e "Anyone not middle class and liberal from North London, Bristol or Brighton are terrifying". I write this because I've heard these views aired completely freely down here in London, often with company, which I find pretty shocking considering those airing these views clearly have a blatant disregard in offending anyone in the conversation. It's extremely disheartening because the same people who hold these views are in the same party as much of the "racists from Sunderland" (all 17 million of them - I didn't know Sunderland was so big) that voted for Brexit, often because their areas have been neglected and left behind for 40 years.
Brexit wasn't a total vote against immigrants (although this was lubrication for the vote), or the Scottish, or anyone else, more so than these downright insulting views from the same people that have dominated socio-economic policy in this country for so many years. It's insane we're still having this argument in 2022 when an A-Level politics pupil could have told you a few days after the vote in 2016 it was socio-economic inequality that ultimately led the vote. It's why I think people who insult Northern Brexiteers as "stupid, racist, ignorant, fascist, awful" or whatever insult you have, just come across as kind of heartless. And why I find it really amusing Scottish nationalists will complain about the past 40 years of British socio-economic policy and vote for something that will ruin their economy, but think themselves morally superior to the English and Welsh,
who have literally the same complaints as them and vote for something similarly economically damaging as Scottish independence because of it.