You may also want to check out some spots around Oaxaca. It is Mexico, but feels more like Central America (to me) - very cheap, people are friendly, and the beach is nice. We stayed at a little hostel on Mazunte beach that was nice for a bit, and Puerto Escondido is also nice (a little bigger and easier to find decent internet.
In Guatemala, Antigua is nice, but can be expensive if you’re not careful. It is a touristy town, but if you can find the spots for the locals you won’t spend a fortune on food. I don’t know much about Guatemala city - there is crime there, though, so you have to be purposeful about where you hang out.
Skip El Salvador. There is a lot to love about that country, but the crime is high, so it is hard to have a nice experience. If you do end up passing through, I had a nice long stay at Hostel Cumbres del Volcan in San Salvador in Colonia Escalon, which was nice. Lots of backpackers passing through on extended Central American trips (though most only stayed a few days. I was there for work, so stayed around two weeks). Playa El Tunco is also a nice spot in El Salvador, I stayed for a week at an AirBnB place that was a yoga studio. PM me if you do end up there, and I can tell you about where I stayed, and probably still have the number of a good driver (which you need in a country like El Salvador).
Again, though, I’ve found Mexico to be a much nicer place to spend time.