I understand where you're coming from as some agents are dodgy. Mine got me set up with a bank account where I then wired the necessary amount and I've been maintaining it with the proper balance for a couple years, and all completely under my name. So I think I'm in good shape.
Why glued to an agent forever? I can choose in subsequent years to use an agent or not. The stamp in my passport is the same regardless whether an agent was involved.
It's kinda nice two options are available to suit people's "best" approach! Good luck. Let's enjoy 🇹ðŸ‡. Some countries make it impossible to stay long term.
Highly recommend an agent. It was the best decision I made. Unless you are extremely bored and want to learn the intricacies of immigration, leave the heavy lifting to an agent who does this all the time. I'm here to enjoy retirement: reduce stress and wasted time, not create more.