For some reason, every author that I know of reaches that phase in their career where they start flexing to kingdom come.
Hey, every author includes us here, and I don't think any of us go out of our way to flex = P. Hecc, my whole thing is writing with such simple language that ESL folk and others who might not be proficient at English can still understand it with ease.
But that aside, honestly, I don't know a single author who has reached a phase where they start to "flex" with a thesaurus. I've seen quite a few authors start off that way since it's what they consider good writing, but I've never seen somebody start off differently and
then start to use a thesaurus.
Now, there is one series I have in mind that's a long-term project of mine which would require the use of a thesaurus... but that's because I want to write it in a sort of old-timey style that makes heavy use of that sort of language, not because I want to flex, and it would only be that one story.
And just as a somewhat related note: personally, I absolutely adore archaic English. I despite overly-complex thesaurus-binging purple prose where people use the biggest most obscure words that they can find just to look smart, but I seriously love old English. A lot of people find some of the characters in FFXIV cringy with how they talk using archaic English, but god do I love it. I just think it's cool and romantic in a way, not superior.