With fantasy and litrpg what makes a first book good for me is the character and world building. In litrpg the main character is typically thrown into a world that they need to figure out in order to survive, a world with "the System," similar to an RPG computer game. The idea of litrpg sounded completely wonky to me but then I accidentally read one (first of the series) and thoroughly enjoyed it (
Dragon Mage Saga) (I also enjoyed his Gods Game series which I read all of).
After the first book the main character is done figuring things out and then it becomes a grind where they have problems to solve, monsters to fight, etc. In some series the exploration and discovery continues into the second and subsequent books but invariably that stuff peters out after a few books.
I also seem to be an outlier in that I don't enjoy reading the battles and fights, which is typically a big part of litrpg. On the litrpg sub on reddit everyone there seems to enjoy the battles, which bore me to tears. I'll always remember how bored I was reading the big battles in the Lord of the Rings, probably 50 years ago.
I often also find the same thing true with regular fantasy series.
Note that this is all very different from thriller and mystery series where each book is designed to be standalone, so that you can pick any one of them up in the airport bookstore to read on your flight. I also belatedly realized that that is what JK Rowling was doing with the Harry Potter series.