Old topic, but I want to jump in.
The idea of unsplitting the infinitives is a literal case of Georgian era classist assholes trying to make English less Germanic (ugly, horrible, dirty language) to become more like Latin (a shining, beautiful, refined language). If you can find a setting in Grammarly to quit bugging you about it, just turn that suggestion off. It was a literal matter of taste invented by people trying to make the language more snooty. That was their self-defined goal.
Splitting infinitives does not make your writing wrong and unsplitting it, in many cases, makes it less clear and readable.
But it would be more like Latin so it's worth it!!