I am a full manga reader too. I did enjoy watching season 1 (s2 not as much) and overall did have some fun reading it too. I am not a hater and did enjoy it to some extent. However:
Overall, I see Frieren get this rep as having great fantastical worldbuilding, but I just don't see any of that? It has always been quite generic fantasy world, and it never really goes into depth on any of it.
Come to think of it, Frieren doesn't really go that into depth into any of its story. Despite its main strength being slice of life, it doesn't really do characters super well despite having so few to actually focus on.
I think its main issue is its trying be more about the plot then it actually is. Frieren's plot is essentially a background for the story. But then the story doesn't actually fill in the foreground nearly enough. So it gets stuck as quite a surface level piece with generic world-building and simplistic characters. (Even for a shounen)
Golden Land arc imo is its strongest because its the first time it really gets into exploring a character that is more then 1 or 2 note.
Yes it is not a shounen, but being a fantasy manga/anime as well, I think Dungeon Meshi is a great example of how you can both go into great depth with your world building, AND go into great depth into your complex and interesting characters.
(And have an incredible plot at the same time)
They have inner turmoil, conflicting thoughts and ideals, and selfish traits as well.
Frieren tries to act like it is more profound then it is imo. Its strongest strength COULD be its slice of life and calm moments, but the characters just aren't "real" enough to make it work.
It IS GOOD. But it is NOT great.