I did it all myself. Of course, if someone is concerned about the time and effort involved, it makes sense to use a visa agency. But as you say......."if" you hire the right people. The number of untrustful visa service agents in Thailand is high.
Why do you think 5 days is not enough? As Stuart Cumming wrote in the previous comment, if you have all the required documents it shouldn't be a problem.
Even when you are vaccinated you can still spread the virus. Don't expect that you can enter Thailand in the next 2 years without having a good reason. Sure not as a tourist.
In any case, you must appear personally at the immigration. An agency will only help you with the documents you need, and the only advantage is that the agency employee who accompanies you can take the fast lane at the immigration.
I used the help of an agency for my very first one-year visa, about 12,000 THB. Since then I have always done it myself.