this is kinda like the tokyo ghoul adaptation, if you watched the adaptation first it was decent, if you then read the manga you'd realize it's shit, I agree with the cutting stuff, but they fully altered the story
the MC is a completely different character, he's meant to be a self-deprecating swordmaster with immense self-acquired talent which he's aware won't mean much when it comes to using it, and we're meant to be shown glimpses of his utterly insane skill acquired by training with the swordsaint in those small moments like the griffin fight, or the last scene of with the blacksmith's handshake, which were all cut out and replaced by moments where he's a complete wuss that just "keeps lucking out" and where "people go easy on him", its :/
its painful because other than the heavy story alteration the adaptation is really good, and the combat is above average too
honestly it's joever for this adaptation, they cut and changed so much, and in my opinion removed some of the best moments, from the fight with the griffin

and the flashback of surena

to curuni's blacksmith scene

to the making of the sword

they are skipping in my opinion some of the nicest scenes with the most character building making all the characters feel incredibly shallow, it's insanely unfortunate, because the animation itself is good and so are the VAs
This is one of the truly brilliant part of the show.
TLDR: It doesn't matter!
The way they made this anime is that you can play the game without watching the anime and vice versa. That said the anime is placed in the EXACT SAME locations as the game meaning if you play the game you will be like "oh shit this was in the game" or if u watch the anime you might go like "oh shit that guy was in the anime" so it really doesn't matter, but you will definitely look at the game differently if you watch the anime first, but neither spoil each other, it's just 2 separate stories happening in the same city at roughly the same time.
Personal preference: anime first, game second as you might get immersed into the game and roleplaying more.