It depends on the genre
Chinese novel urban category with a singer MC is probably one of the easiest to end
Because at 1k chaps, MC is most likely globally famous, is with FL, and probably retire.
1k chaps is also a studied number for chinese novels. Because more than 1000, the reader feel the author is gonna pad it and turn it into a trash novel, and less, it feels that author rushed it as lot of novels had 1k chaps end.
There is a sci-fi novel with the (kinda, kinda not) cliche plot of one scientist that must stay awake 1 day every year, while the rest of humanity is in cryo. The destination got a precise number of years. So the end is already known it gonna succeed.
There is difference between "writing on the fly, chapter per chapter" and "i already envisioned everything and i fill the insides".
The biggest advice though : just do what you or your readers feels right for the ending.
There was a very good chinese novel that was destroyed by it's ending, because no clue what kind of stuff author sniffed, but he made the MC being stuck in the eternal sun or something, with eternal pain, and his pet that feel sad for him, make him re-see his life to appease him, but in fact hurt him a ton as his life was also hurtful quite a lot with regrets. So after chinese readers riotted in comments, he deleted and rewrote a good ending, but for everyone the shitty ending is basically the "canon" ending.
One thing you must pay attention, is that after a series end, all authors in the world are never sure they can make another good series.
And that is why they can continue a long time a series, even milking it.
To be honest, it's genius like "Andur" from Royalroad, that have a clear plan for each of his novel (that are in fact connected to each other), and a high-quality writing, the ones that dont fear about endings.
Because like "hades" game devs, everything is probably clear for them and focus on "next one".
There is a interesting thing that a jp WN became popular AFTER it's ending.
Yep it's Arifureta. This not-so OLD webnovel (anime was recent and disliked by true fans) somehow had it's ending done where protag come back to earth with his harem.
But somehow author decided to write "after-stories" chapters. Where it show MC+harem (+classmates alive) living on earth.
Thing is, he also did some arcs to add more depth to some characters (for example the masochist dragon woman/the teacher waifu) or what it feels like to be a hero back on earth.
But a few years ago, readers didn't seen often series with a isekai protag back on earth, it attracted lot of fans to it. Me being one of them.
And to be honest, the quality is over the top, because the switch of environment, give lot of opportunities. And some particular plots happen which totally don't follow the rythme of the "normal plot".
Still today, i think author is still continuing to write "after-stories". In fact it can be said it is no longer "after-stories" but more like "a sequel" XD
PS : after-stories begin at ch176 i think