Client management system recommendations?

I used to have a small WordPress-centric web development business, but due to a mixture of family circumstances and losing a lot of work at the start of COVID, I shut it down and got myself a regular 9-5.

5 years later, and I'm plateauing at my day job - there aren't any real opportunities for advancement or increased pay... So I'm thinking of dipping my toes back into building websites again - albeit on a smaller, part-time basis to start with.

My original brush with the industry was almost accidental. A hobby that naturally turned into a business, and because of that, it wasn't well planned or organised. I want to do better this time.

One of the biggest challenges I faced previously was managing client payments/subscriptions and taking sites offline when payments were missed. I did this all manually, which was fine when I only had 20 clients, but when I had 200, it took a considerable number of hours each week. I'd like to automate this.

Does anyone have any recommendations for the best way to approach this? Extra points if it's Wordpress-based, as that is where most of my experience lies, and it's also the area my current day job revolves around.

Thanks in advance for your help!