Central America Touring

My parents used Caravan Tours see Guatemala a few years ago and they really enjoyed their trip.
https://www.caravan.com

Costa Rica was great, but not sure about Guatemala, Honduras and El Salvador. How safe are those countries?

Age group? Level of academic interest? I think you need to tell us more, Paul.
Roads Scholar for older set comes with evening lectures.
Overseas Adventure Travel is 50+ but often 60 or 75+. No single supplement so more solo travelers. Small group. A travel aquaintance used them for Central America and enjoyed the trip as he was interested in cultural history.
GAdventures, Intrepid or Exodus are global adventure travel small group companies. Global but HQ based in Canada, Australia and Britain respectively, so may skew to that demographic a bit more. All offer various comfort and activity levels, and some to specific ages. The more comfort and cost, the older the average traveller.
These are all the companies I would search first for my tour research.
But you may consider flying between major cities and organizing a private bespoke trip with a local company. I expect their ecomony would appreciate it.
As to safety, mentioned above, any place can have political flair ups, but in general travel is safe as long as you ask where you shouldn't go particularly at night. Any basic web search of 'where tourists should stay in XX (city)' will tell you that, and I have started to read expat websites for further info about a city. And of course, if you stick to a tour, no self respecting company wants to land their clients in trouble.

We used Caravan and Overseas Adventure too for Costa Rica. We traveled in the spring so no young kids or teenagers, but there certainly were 30, 40 and up your members.
I avoid other Central American countries for several reasons.