When you first land you'll get a 30-day visa, then the agent can get you a 60-day extension and then after that you get a 12-month extension. That can all be done on the same day with the help of the agent so your first retirement Visa has a total of 15 months. Yes you can do it yourself but many people choose to go with an agent because there are so many different versions of paperwork that you will need and all the different areas Within immigration office have to be visited. You are paying for their expertise and their experience just like you would if you paid a lawyer or an accountant. You are paying for their experience
If you prefer to do everything yourself including going to the immigration office and and making sure you have all the necessary paperwork in order that I would recommend doing it in the US before you fly here. If you want to trust a company to do all of that for you then talk with an agency that specializes in immigration. They will tell you to do it once you get here. I used it agency and they took care of all the hassles for me and I am glad I did. I would rather spend the money and have a professional do it for me.
Best and affordable are two different things. I have found that affordable agents can't get things done as well as other agents that cost more. If your goal is to get things done right the first time and not have to worry about it then that it's going to cost you a little bit more
I use a free VPN (Proton VPN) that is connected to a number in the US, sometimes you have to keep changing servers to get a US server, then I use Google voice and it's free. Both items are free