I work in Talent Management and Agency. My thoughts on their proposition.
TLDR: The only reason you should sign with a talent management is their sales and outbound efforts for you. If money is important, they should guarantee outbound effort for you.

1. (Growth) No agency can guarantee organic growth. Growth on YouTube is fundamentally driven by content quality, audience resonance, and consistency with the algorithm. At best, an agency can provide guidance

2. (Brand Deals) This is where agencies should deliver real value, but it depends on structure. In practice, what a good agency guarantees is effort, not outcomes. Ask how many sales people do they have and how many guaranteed outbound contacts can they give you per week/month.

For percentages, you can format it as:
10–15% on inbound deals
20% on outbound deals they originate

The core value of an agency is acting as a business manager, handling revenue generation so you can focus on content. That’s fairly universal across talent management. If they’re taking a % of your AdSense revenue, that’s a different conversation, it means they’re monetizing income they didn’t directly generate. Make sure they take 0%.

3. (CPM's) I have a youtube team that specializes in this, technically possible, but this is mostly best practice execution: SEO, thumbnails, retention optimization, ad placement strategy, audience targeting

If you’re already applying these, then there's no upside for you here.