I don't think I can answer your question.
But I know it would be better if you provided more information.

@bverdoorn said:
I created a list of 800 movies

That list you created, what data does it contain? What fields?

and I would like to download the basic fields (id, title, release_date, overview, genres, popularity, etc.) via the API.

Which fields do you want the data? There can't be etc ... there.

I was able to get 500 records out,

How did you get the data for those first 500?
And how is this data stored?
A single table? One .NFO file for each movie?

Google Translate

The list was created in TMDB, The 500 records that I downloaded via the API into Python contained the following fields (vote_count, popularity, id, video, media_type, vote_average, title, release_date, original_language, original_title, genre_ids, backdrop_path, adult, overview, poster_path, original_name, name, origin_country, first_air_date). These were the fields that were delivered through the GET /list/{list_id} method via the API. Thanks for the quick reply.

I personally never used this feature.
But I believe that using Lists in V4 mode it is possible to import an unlimited amount of items.
What I don't know, because I don't use it, is whether there is a quantity limitation when making an GET API Request.
But I think it’s worth taking a look.
https://developers.themoviedb.org/3/lists/v4-or-v3-lists

Yes, the best way to accomplish this will be to page through the results on a v4 list. This way you can get all of the items.