You don’t have to own the projects. Develop on the free plan. You can then transfer the project to the clients org. Then you can just get added to each client’s org by the client. So you just pay 25$ and can be added to any number of projects because the client is paying for them.
Why create separate accounts when you can transfer projects. Build on your own account then transfer it to the clients account. Clean and I guess transfer option is made for this purpose.