You are totally right!
I’m halfway through and it’s wonderful. Too bad Viki doesn’t have this drama, it’s really great till now.
(I especially like that they show all the characters as multi-dimensional, every bad character has good qualities, every good character has bad qualities). OST is also great.
To see a reprise of your Kings - Jang Youngsil - there’s places to watch that around. 
Also has the STAR of JUMONG and his triplet children.
also it’s about a great man - sigh. A slave who definitely helped the Great Sejong.
So many great dramas…I could watch Sejong again.
I am so late to the party!
Up to a year or two ago I would have said I AM the Queen.
I had watched everything Korean! But mow…?
It also depends what you all a Sageuks? I have a long list of most of the dramas I have watched as well. However, to me the golden age of Sageuks is becoming lost and replaced by fluffy rom-com that they just put in the Joseon period without much substance lately. Also, for example I wouldn’t call Queen In Hyun’s Man & Rooftop prince Sageuk’s—too much of storylines take place in modern times. The changes in Korean Sageuks have been so great I have drifted more towards historical Chinese dramas. Though K-drama modern rom-com or melodramas are certainly better than their C-Drama counterparts. The last Sageuk I really loved was Empress Ki and liked off the top of my head the Grand Prince. While Jewel in the Palace is probably my favorite classic Sageuk.
I’ll try to post a list tomorrow.
That’s a question I find hard to formulate an answer to. I think, for me, it just depends on the feel of the drama.
So, I read a bit more about what classifies sageuks, and sageuks tend to be categorized as
Time-travel and fantasy are big themes in fusion sageuks, so you could classify rooftop prince and Queen In Hyun’s man under that category. The golden age of Sageuks was filled with a mix of authentic, fusion and faction sageuks, for example Empress Ki would fall under faction on the surface, but some people argue it’s a fictional sageuk (the person was real, but none of the storyline was).
I also wonder whether I would classify Mister Sunshine as a sageuk, or if that happened ‘too recently’ in history. And dramas like Arthdal Chronicles would fall in the fantasy category for me, even though I would classify The Gu family book as a fusion sageuk.
TL;dr - It’s personal.
I’m having a lot of fun catching up on dramas I haven’t watched before from the ‘golden age of
Sageuks’ 
The setting of Mr. Sunshine is more than 50 years in the past during the years when Japan began the annexation of Korea—this would probably qualify as historical fiction.
Although Mr. Sunshine is a historical fiction, Koreans would not consider that a Saguek.
In Sageuk, you’d expect to see kings, queens, princes and princesses and possible concubines as well as court ladies, royal guards and eunuchs, and all those high and low ranking court officials! Oh, yeah, and don’t forget the maids and servants who were born into those positions.
There’s a 1999 and a 2013 version, right?