If you want to use an agent, you should already be in Thailand for this type of service. Also very helpful if you say what area of Thailand you're going to be in, as agents are local to a city/prov and each immigration office can have somewhat different rules. Many things currently in the process of being updated/changed. Some things reported to be official and published in the coming week... for me I'm waiting for the official announcements vs news speculation. Hopefully mostly positive improvements coming
There's BTS and MRT, separate companies, separate stops, similar prices. There's also a few routes on the canal boats for a different adventure. Sometimes they have all you can ride tickets for several days, you might look into that.
So you're saying not many people get visa within the country. From reading various websites and experience over the years I find that highly unlikely..also many on here say doing it after arriving is easier. Bit again if that's what floats your boat feel fee. Walking past jomtien soi 5 a few times a day for months will just write off the masses of people surrounding the immigration office all day as an illusion. Maybe that's why they're wanting to ban all the pot shops everywhere due to mass illusions from passers by.