Three fair questions, Tera.
1. Tyrannical government is one that does not seem to respond to the needs of the people. A properly functioning constitutional representative democracy (as opposed to "true" democracy) as defined for the USA tries to represent everyone's as much as possible. One could easily argue that until we got rid of legalized slavery, we were failing in that regard. (No opposition here.) One could argue that until we enabled women to vote, that we were failing in that regard. (Again, no opposition here.) The "Black Lives Matter" movement, as far as I can tell from what I have seen, heard, and read, is NOT about USA governmental oppression of blacks, but of SOCIETAL oppression.
You judge a government to by tyrannical when enough people have had enough and successfully overthrow the tyrannical government. You are quite right that it is a relative, not absolute, viewpoint as to where tyranny starts. You would need a history expert or a good review article to enumerate the actions of George III of England with regard to the colonies, but he apparently crossed the line 'cause we fought - and won - the war.
2. Not all countries in the Americas had the same situation because some learned from our examples. Not only that, but we had multiple countries populating the colonies. Canada didn't fight a war of independence from France or England and many people stayed allegiant to the European founders. Many of the Spanish colonies in North America split from Spain. Mexico did too, eventually. But (with due respect, Tera), other people's experience in the colonies is irrelevant. We experienced what we experienced, or to paraphrase President Trump, "It was what it was." But to answer the question about gun laws in other countries, ...
https://en.wikipedia.org/wiki/Overview_of_gun_laws_by_nation
3. The U.S. Constitution is indeed a living document and might one day be amended regarding gun laws. However, it takes a lot of societal pressure to do that, and we have had experience with changes that don't work so well. The 13th amendment abolished slavery. The 14th Amendment extended the "Due Process of Law" concept. The 15th amendment established certain voting rights. So yes, it changes from time to time. But we also saw what happened when some people tried to legislate morality with the 18th amendment (alcohol prohibition) in 1919, and by 1933 the country ratified its repeal.
The question remains, will the USA ever feel safe enough to not keep guns? But if you look at world events, where thugs from Boko Haram kidnap children to keep them from getting a secular education, that would scare parents. You look at terrorists using IEDs and bomb vests in some areas. We have been attacked by terrorists ourselves. When you see riots in the streets of your own home town, as has happened in Detroit, Portland, Seattle, Los Angeles, Boston (we are talking over the last several DECADES), you see police stretched to their limit and unable to respond quickly (or at all, in the case of the Seattle "CHOP" area). You see rioters tearing apart businesses. And you want to tell people "Sorry but we can't protect you ... and oh, by the way, we are going to take away your guns so that YOU cannot protect yourself either." All I can say is "NO." That just isn't going to work here.
I don't know if I have adequately expressed your questions, Tera, and I would be happy to explore the situation further. However, I must admit that past a certain point, I am no better a source than a good web browser would be. It has been 55 years since I studied US History in school.