I had similar problems, but on a much smaller scale. I thought I had a good relationship with one client and that invoices were being paid on time, but then I found that around £10k had been overlooked by AP over the course of a few years.

After that I phoned after every invoice - which was a pain in the ass for me and for AP, but the money (mostly) arrived on time.

It's really, really easy to miss how important admin and cashflow are. They're boring AF, but also too important to ignore.

Even worse is the realisation that sometimes you are going to be paid late, sometimes you are going to be offered partial payment, and sometimes you may never be paid at all - and these are all absolutely normal in business and you should expect them and have contingencies for them.

As a rule, any freelance work where you rely on timely payment from a single client to cover your outgoings is incredibly risky and dangerous and can easily become very stressful - because you are literally flirting with financial death.

It's not a mess unless you have some weird hangup about billing people and agreed-upon rate for an agreed-upon service. I freelanced full-time for 3-4 years, at one point with dozens of active clients, and never missed a billable hour. It's not difficult if you have a system to track and bill. Yeah it takes time, and it's a grind, and there are times you'd rather just not do it, but to imply that freelancing makes it easy to miss hundreds of thousands of dollars in billables, which is a non-trivial percentage of most folks lifetime income, does not jive with my experience in the slightest.

You did 3-4 years with dozens. I am at 20 years and more complexity. For a number of years, I managed staff and interns - just keeping interns busy chews up a lot of time. I had a co-founder to start with, and was then solo running it from then. Once you have staff but not the scale to get help managing that, it gets seriously difficult. When I started, things like Toggl didn't really exist. I used to print out and manually sum hours from staff per client/project and per month. And then create dozens of invoices each month manually in MYOB, an awful and miserable interface. It was a huge timesink. I couldn't outsource it because I was the only one with true knowledge of each relationship - what to bill in what way, which clients were sensitive, etc.

I'm not saying it's impossible to do it OK, I'm saying it's easy to get it wrong and fall behind. I mentioned the idea of an opinionated business framework to a friend (freelancing very occasionally 1-2 years) and they said "Oh YES, absolutely." And that's for someone who'd send 2-3 invoices a month, not 50+.

It's not just about missing invoices. Freelancing didn't really satisfy me to be a one man business army. Having dozens of active clients sounds like a horror movie to me. I also worried much more as a freelancer than as an employee.

It's great that you could do it.