The strongest open-source Generic Crossword Game* program that I know of is Quackle. The page has a link to their github if you want to check it out. It's been in development for almost 10 years now and was originally started by two Google devs who happen to be among the highest-ranked players in the international tournament world. Its difficulty levels are basically just determined by simulation depth/speed, and its heuristics are based on expert-level strategy, so even at its lowest setting it's probably unbeatable unless you're a strongish tournament player. Many players use it primarily as a post-mortem simulation tool and endgame analyzer.
There's a reasonably large community for contributors and players on Yahoo! Groups if you want to drop in and ask about the project. A lot of the strategy discussion might be daunting when they get into stuff like leave values, tile synergies, etc. but most of us Scrabble geeks are pretty welcoming and enjoy explaining the game to newbies. Just don't ask about US vs. International English dictionary -- it's basically the systemd debate of the Scrabble world. 😫😫😫
*Hasbro and Mattel are notoriously vigilant about protecting their copyright, which is why you have to designate the placement of the premium squares when you first compile. You can play with whatever board layout/square values you dream up. Something to keep in mind if you continue to grow this project.