It’s exciting when the client work starts to take off but you’re right to strategize how to handle multiple projects! I’m in the same boat as Adam, and am limiting how many I can realistically handle at once. I have notes in my project management system to start up with some new clients three months from now.
I use 17Hats as my project management platform, using it mainly to send proposals, contracts, and invoices. They do have a workflow function where you can set up workflow templates for different types of projects. Sometimes I make a totally custom workflow; depends on the complexity of the project . They are glorified to-do lists, but keep me organized as I know right where I left off on a project. I also use the notes function to keep track of all the details.
Lots of folks here use Dubsado too, but you can certainly replicate all this with Google Docs/Sheets for free! Both 17H and Dubsado usually have good discounts around Black Friday, just FYI.
Good luck with your return to work and getting some more clients!