There are probably modern Isekai that are well written and does something good with the ideas but are buried between the sea of mediocre slop
Probably. It likely doesn't help when it's generally more difficult to find interesting stuff as a westerner unless you're extremely in-the-know, so we just see stuff like Shield Hero on the front page of a streaming service and think that's all there is. I'm guilty of that too, but I still have issues not pertaining to the setting itself.
more and more of them are adaptations of Light Novels instead of original concepts
This being one of the main ones, for me.
Anime is mostly used as a marketing ploy for the original material
That's also true.
It's a gigantic medium, so it can be tough to differentiate on what's more of a tie-in, and what's more of a wholly original work (both of which can be good or bad, obviously).
I prefer reading mangas in general over watching an anime adaptation (since I'm an animation enthusiast, not necessarily a consumer of cartoons in that sense), but that's because I like reading all kinds of comics.
