Either way congrats on starting your own agency! I was an early employee at an agency (not focused on politics but likely similar to yours in other ways) and when I started out there were fewer than 5 employees including two founders. It was very top-led in the beginning, but as we grew to 20 people or so that became unsustainable.
At that point it became about building a system instead of trying to get everyone up to the same skill level. At a basic level we decided that we were really working with clients to set expectations and then meeting (and hopefully sometimes exceeding) those expectations. We too had issues with last minute changes, until we changed our contract and statement of work language to specify the framework for making decisions (usually budget/spend/creative related etc) and how those decisions/changes would be implemented. Same thing with mistakes: part of the expectation from both sides was built in time for QA to catch mistakes before they happened. The more you can set up the framework for success early on in the sales/pitch/onboarding the better. And clients will probably like you more for setting up reasonable guardrails in the beginning.
The framework also allowed for some influence/involvement from the top, which was helpful in filling those inevitable skill gaps. We used some version of the framework until we got to about 45 employees, and then it had to change significantly again to fix some of the bottlenecks that were coming from the top down. It also opened up more time for people at the top to work on business development and sales.
I left out a lot of details but overall it took a “systems thinking” mindset to fix issues once we got to a level where the top-down approach was no longer feasible. The system was set up from top down, but once implemented it could basically run itself with a few tweaks throughout the years. Maybe you can focus on setting up those systems instead of trying to level up every employee to be a mirror of you and your partner.
Hiring/firing when there is a strong system in place also seemed easier: look for people who can grasp/appreciate the system and their place in it, give them agency and opportunity to grow within the system, and evaluate them based on the system and the expectations it sets. We almost never lost talent to other agencies and we rarely had to fire anyone. Maybe we were just lucky, or maybe the system did its job!
My two cents on growth/profitability for an agency: The existence of other agencies, however inferior they may be, means there is a limit to how much of a premium your agency can charge. At some point someone with control over the budget is going to balk at your price and go with the competitor who is also saying they can do everything you can do. Which means at some point you will need to grow your client (and employee) roster or be satisfied with stagnating profits.