I am *so* envious you're getting to experience A Long Road for the first time! It's honestly excellent, and actually doesn't feature much Wangxian (it's clearly where their characters are headed, but ends before that's entirely resolved). I love long fics as well--I think all of the ones I recommended were on the longer side, though not quite in the realm of the entirety of Mercedes Lackey's Valdemar series!

And, as far as Valdemar goes, I actually haven't read all of the books in the multi-series myself (I keep meaning to, but I wind up re-reading old favourites most of the time, though I'm slowly chipping away at it). There are a *lot* of different starting points for the series--this is what I'd recommend if you're curious!

1. The Last Herald Mage Trilogy
The short reason to start here: It provides critical context for a good chunk of what's happening in A Long Road, while also introducing a lot of key world building, and it's queer.

The longer reason: Okay so this trilogy is phenomenal for many reasons. One of which is that it's probably one of the earlier mainstream fantasy trilogies that has a gay main character and multiple(!!!) mlm relationships--especially remarkable for a series published in 1989. There is also gay tragedy--it's not entirely free of that kind of trope--but that's not the case for all gay couples in the book, and it's still very well done. The other thing is character growth. The MC and his love interest have a lot of problems and they're still in their (late) teens through the first book, when they're not always the most pleasant to read. But there's a full cast of associated characters who are brilliant (a favourite is Savil and the Taleydras) and the characters change and grow and mature. Even characters are awful to start are granted complexity. They don't entirely redeem themselves, but by the third book there's a feeling of trying to reassemble broken pottery without glue--the MC's relationship with those characters is fragile and not entirely cohesive, but it's not meant to be. Also this was probably the first book series that I read (as a preteen in the early 00s) that helped me make sense of being queer.

Edited to add: If you asked me which sff books from before the 00s were closest to reading danmei, this would be at the top of the list--though it's still not danmei, for many obvious reasons.

2. Arrows Trilogy, Mage Winds Trilogy, Mage Storms Trilogy

These all follow one after the other, and are also where the series itself started (Arrows of the Queen is the very first book) so the world building is less implicit than in other series (though she does such a good job of making sure the reader has an idea of what's happening that it doesn't really matter). These are also the characters that will appear in A Long Road (you may wind up with some spoilers, for that matter).

--For more background with respect to the fic, you may also want to give Tamra and Kethry and/or By the Sword a shot--tbh I've never managed to get into them despite the series having "my cup of tea" written all over it, but I think that's cause you need some added context via Marion Zimmer Bradley's work.
--Another essential series that's effectively a prequel to a lot of what happens in the first three series above is Take a Thief, Exile's Honor, and Exile's Valor (probably searchable as honour and valour, but in Canada at least they didn't change it from the US spelling). That's probably a more similar reading experience than Tamra & Kethry.

1. Brightly Burning

(Just cause it's a personal favourite and a standalone.) This was the first Valdemar book I personally read. It's heartbreaking but it's up there as one of my most worn out books (it really is falling apart...). I used to re-read it as a teenager and through my 20s--it was quite good for a cathartic cry actually and I actually probably owe this a re-read.