Notes

https://www.youtube.com/watch?v=bLW-WbUgiuY
18:30

First things first:
The series was filmed under the budget of 6.5 million dollars (18.5 million złotych back then), the filming took place between 21st of July 2000 till 14th of March of 2001. That's 236 days and that's a lot (but they were filming through the seasons, so go figure). They were supposed to wrap this up by the final week of January, but the weather was uncooperative, and they both needed a lot of snow AND get out of the places where they got snowed-in.
The best source of information USED to be the OLD official page of Andrzej Sapkowski, which had a separate movie/series section, with extensive materials from pre-production, production and post-premiere. It included bunch of very informative interviews, most notably with the director, executive producer and the scriptwriter (actually the same guy - writer was the exec). Not a lot of that stuff survived when the page got updated in 2010. Another good source of information was the on-set documentary they've filmed as part of the promotion gig. It shows the sort of attitude various people in the production had toward it. Other than that, it's just dredging the net for old articles, forum posts and what not. I'm actually glad that the official English title is Hexer, as that makes looking for things so much easier (you have no idea what a nightmare it is to dredge for it in Polish, where everything is under just Wiedźmin)

Bunch of notes, the post is probably going to be super-long, so rudimentary time-straps:
8:40
What you are missing on the effect is not as much the budget, but the year. This was shot in 2000, and edited in 2001. Go look for The Mummy, the 1999. It's a great adventure movie, with some of the most ground-breaking CGI effects when it came out... and try not to laugh watching it today. And that movie had a budget of 80 million dollars. Hexer was made ad 6.5. And Dragonheart was done on the budget of "only" 57 million, but literally 40 of that was spent on the dragon and another 17 on Sean Connery to bother working in the movie. So Connery took more money for voicing a CGI dragon than they had to make Hexer as a whole
10:00
I covered it in a comment under the shorter version of discussing the series. This time you are overthinking it waaay too much. The katanas (because that's what they were intended) are a combination of three factors: they look cool and it's the late 90s, early 00s (1), they run out of money to hire actual fencing masters to help them with fencing choreography (2) and they already had an aikido master as a consultant and stuntman (3). So basic aikid sword practice, aiki-ken, was grafted into the production, even if aiki-ken has ZERO combat value (it's essentially a balance training exercise), and the logic to that was that "if Geralt is a killing machine, why not focus on the machine part to make it extra cool, even if we don't have the money". While it goes contrary to the combat as described in the books, Sapkowski always jokes that he himself has NO CLUE about fencing and just wrote what he thought was cool, so it's not some big sacrilege here. This ALSO ties with the prevailing Polish fantasy aesthetics of that era, which were borrowing left and right from both Far East (not just Japan) and steppe nomads. Just google artwork for "Kryształy Czasu", the Polish TTRPG of that era. Hence it was adding to the "exotic fantasy elements" and how witchers stand out
11:50
I actually checked between previous video and this comment. The reason why they went with a quarry was because they planned to use Ogrodzieniec castle ruins (google it), but didn't get permission. And Ogrodzieniec is a pretty badly battered ruin, so it's not exactly useful by itself, as pointed out. But the goal was to use the fairgrounds at the bottom of the hill with Ogrodzieniec in the background, the sort of "this was our castle, we're now in the caves below it". It played the role of iconic ruins in so many movies made in Poland, it's actually GOOD they didn't get the permission.
14:30
Scouting was notoriously dodgy. In fact, the guy responsible for scouting got fired after the production, so that's some sort of consolidation.
16:30
Copyrights. It was all about copyrights and messed up (fucked up, even) negotiations with Sapkowski, where on one hand, the producers wanted to get them unreasonably cheap, while he wanted to sell them unreasonably high. A kind of bidding and rope-tugging went back and forth for a while, and they essentially bought bits and pieces out of him before he walked out of negotiations. This is easily one of the most important factors affecting the production. His idea was to troll them, selling them "unfilmeable" stories. As to why insist on the dragon - it was evisioned by the head producer (not the exec) as this great giant central piece to be part of the movie. To which the exec said "Wait... what movie?"