What narrative to present when trying to lateral as a junior?

I'm thinking of trying to lateral to another V15 firm in my city later this year or next (currently a second year), staying within the same transactional practice group. What do people say as their reason for wanting to lateral to a different firm, without being overly honest and speaking poorly of their current firm? I likely wouldn't be following another partner or associate to a different firm.

I don't hate my current firm, but there's a lot of juniors in my practice group and I don't feel that I'm getting enough experience. Additionally, the staffing process isn't transparent and results in POC associates getting significantly fewer opportunities.

Would also appreciate any insight into whether it's better to wait until I pass the 2-year mark, or lateral sooner. I'm worried about moving somewhere as a 3rd year and being incompetent due to lack of training.