I actually liked The Last Stand. Its similarities with X3 aren’t what bother me - it’s that they Git away from their roots. IMHO - one of the biggest strengths of the X-men movie series is how it resonated with themes and issues relevant to their time - racism, discrimination, prejudice. The first few movies (especially X2) kind of “Star Treked” big social issues like gay rights when they were a very hot issue (not that it isn’t now but there was lots of back and forth with “gay marriage rights” vs protection of “traditional marriage” via state laws and appeals. They had to - superhero movies hadn’t become as mainstream yet so they had to really hold back on a lot of the trappings of that genre.
To me, the later movies, like apocalypse and DOFP didn’t seem have the same kind of resonance to current issues. At best, they were good superhero movies. But with Marvel already doing good superhero films pretty successfully, X-men movies just didn’t seem as special. Adding the mutant cure into X3 at least kept that film tuned in to the social issues and themes set up in the first two films. The Phoenix sagas were some really good stories - but they didn’t offer much in the way of parallels to relevant issues the way the first two films did.