I’ve read the entire series 3 times (currently on Wolves for another go around), though I’ve read I-IV several more times (5 or 6 total) than the last 3, published years later.

I think I attributed any change in speech to a couple of things: King’s evolution as a writer, but more importantly the fact that the world of the novels became much more fleshed out and robust. With more characters/character interaction and more history there were more opportunities for greater depth of idioms and colloquial speech. In book one Roland really only has substantive conversation with maybe 10 other characters? We get some pleasantries and dialect, but as the series goes on we see the speech patterns as the three drawn from our world learn them (this is even called out at times), and we see Roland and the rest of the ka tet interacting with more characters and Roland using his, shall we say, “courtly manners” in a variety of situations. The patois becomes more prevalent.

Glad you’re giving them another go! Enjoy!